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Biological Response Modifiers in the Management of Viral Infection
Viral Diseases
0195-5616/86
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Biological Response Modifiers in the Management of Viral Infection Richard B. Ford, D.V.M., M.S.*
The year 1908 seems to have been a hallmark year in the history of therapeutic immunomodulation, perhaps best referred to today as immunopharmacology. In that year, Calmette and Guerin, while working at the Pasteur Institute, developed an effective vaccine against human tuberculosis using an attenuated strain of Mycobacterium bovis, the Bacillus Calmette Guerin (BCG). 41 Also in that year in Holland, Paul Ehrlich suggested that the body contained intrinsic "protective devices" capable of causing aberrant cells to remain latent for decades rather than to develop into cancer. 29 This early concept on the theory of immune surveillance has since been extensively elaborated on and is probably one reason that biological response modifiers (BRMs) are receiving attention today. It is interesing to note, however, that only during the past 40 years has our knowledge of immunologic responses progressed from the somewhat rudimentary description of intrinsic host defense mechanisms to a more comprehensive understanding of cellular and molecular processes involved in the immune system. It has only been during the past 20 years that investigators and clinicians have gained an appreciation of the role certain agents have in augmenting immune responsiveness. As several new waves of research attempt to narrow the gap between experimental observations with BRMs and clinical therapeutic trials, the concept of a pharmacology that can modify and even control immunologic function continues to emerge. Although in its infancy in human and veterinary medicine, the potential role of BRM therapy in managing cancer, autoimmune disease, infectious diseases, and allergies continues to generate considerable interest. Clearly, the concept of altering a disease process in a given patient by administering chemical or biological agents that enhance immune responsiveness has surpassed theoretical acceptance. Human clinical trials, particularly in cancer patients, involving BRMs are widespread today. In contrast, relatively little information is available on the clinical applications of immunomodulating *Diplomate, American College of Veterinary Internal Medicine; Associate Professor, Department of Companion Animal and Special Species Medicine, North Carolina State University School of Veterinary Medicine, Raleigh, North Carolina
Veterinary Clinics of North America: Small Animal Practice-Vol. 16, No. 6, November 1986
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agents in veterinary medicine. 4 •14•15•16•31 •34•35•36•48 •58 Despite the fact that BRMs are not in widespread clinical use in veterinary medicine, their existence and limited availability are gradually becoming known to practitioners, as well as clientele. With time, information about these products, as well as their therapeutic claims, will undoubtedly stimulate interest among owners of pets that might benefit from such therapy. Compared with the extensive volume of literature addressing the role of BRMs in the treatment of cancer, the number of reports that address the role of these agents in the treatment of patients with infectious disease, particularly patients with viral infections, is relatively small. Nonetheless, such products do exist and are currently being marketed in veterinary medicine. Unfortunately, there is far too little information available on which to judge clinical efficacy of the products currently marketed in the United States. The intensive investigations directed at finding a cure for acquired immunodeficiency syndrome (AIDS) will undoubtedly result in the development of several immunomodulating agents, as well as antiviral compounds, during the next 3 to 4 years. As immunopharmacology continues to emerge as a recognized clinical discipline, many new and more effective agents will undoubtedly be introduced into human and veterinary medicine. The burden of responsibility ultimately falls upon the clinician to understand the clinical indications of these agents, as well as to critically assess therapeutic efficacy and safety of immunomodulatory therapy. In the treatment of cancer, BRMs have been introduced on the premise that cancers develop within the framework of immunodeficiency or at least of defective immune surveillance. In addition, tumors are antigenic and, if reacted to by a stimulated immune system, would either regress or not recur following removal. This, in addition to the fact that cancer is generally associated with progressive immunodeficiency and is compounded by immunosuppression associated with other therapies explains the current level of interest and research into immunomodulatory therapy. 33 Biological response modifiers do have a role in the management of viral infections, particularly in infections caused by viruses capable of inducing immunosuppression. However, the precise role of these agents is less clearly understood and has not been investigated to the same extent as BRM therapy in cancer. The intent of this article, therefore, is to review the BRMs that have reported efficacy in the treatment of viral infections and to summarize their mechanisms of action and potential clinical application. The discussion that follows focuses only on BRMs currently under investigation and on those that have demonstrated some potential, either in vitro or in vivo, for augmenting immune responsiveness in animals with viral infections. BRMs DEFINED Also referred to as immunomodulators, immunoaugmentors, immunoadjuvants, immunorestoratives, immunopotentiators, and immunostimulators, the term "biological response modifier" refers to individual agents, either biological or synthetic, that are capable of eliciting specific and/or
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
Table l.
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Categories of Biological Response Modifiers and Representative Agents BIOLOGICAL RESPONSE MODIFIERS USED IN IMMUNOTHERAPY
"Prohost" Agents Lentinan Tilorone Lipoidal amines MVE (maleic anhydride divinyl ether)
Immunorestoratives Passive immunization with gamma globulin Plasma infusion Immune serum globulin Interferon Thymic hormones (thymosins) Marrow, thymus, fetal tissue transplantation
Noncytotoxic Immunosuppressive Agents Cyclosporins Apheresis*
Immunostimulants Dialyzable leukocyte extracts Transfer factor Levamisole BCG Propionibacterium acnes (syn: Corynebacterium parvum) Tuftsin Bestatin Isoprinosine
*In contrast to plasmapheresis, apheresis refers to the process of withdrawing a patient's blood, separating, and removing specific constituents, such as plasma, huffy coat, or platelets, prior to reinfusing the desired components.
nonspecific effects on immune responsiveness. Clearly, the role of BRMs in "prohost" therapy has been emphasized. "Prohost" therapy refers to those agents that augment the cellular immune response and may ultimately lead to an improvement in the overall health of the patient. Such agents may be further subcategorized into those that facilitate a normal immune responseimmunorestoratives-and those that stimulate the immune response-immunostimulants. Another group ofBRMs is capable of inducing noncytotoxic immunosuppression. 7 •19 Such agents are currently being studied for their role in the treatment of autoimmune disease and in tissue rejection inhibition following organ transplantation (Table 1). Some degree of immune impairment is associated with most disease states, particularly systemic disorders such as cancer, autoimmune disease, and infectious diseases. Therefore, the assumption that patients affected with a systemic disease would benefit from therapy that either restores immune responsiveness to its normal "resting" state or that augments the normal immune system seems logical. However, clinical trials with BRMs in humans and animals do not consistently support this premise. The assays used to establish possible efficacy of a given compound as a BRM are largely based on in vitro test systems and, as such, have generated greater expectations than can be produced in clinical practice. Perhaps this is attributable to the fact that clinical therapeutic trials are commonly carried out on patients with overwhelming infections or during the terminal stages of a disease. Yet the clinician, as well as the investigator, must contend with numerous other variables in the living animal, including the host-virus relationship, the impact of other therapies on immune responsiveness, and the function of immunocompetent cells.
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BRMs USED IN THE TREATMENT OF VIRAL INFECTIONS A universally accepted system for classifYing BRMs has not been developed. Generally, most discussions on the role of BRMs in cancer management and therapy either do not attempt to categorize individual agents or segregate them by chemical classification or mechanism of action. For purposes of this discussion, BRMs that have at least potential efficacy in the treatment of viral infections are segregated into three broad categories: biological agents, hormonal agents, and chemical agents. These categories and the agents within each category are summarized in Table 2. Although all of the compounds listed in Table 2 have documented antitumor properties, this by no means suggests that all BRMs used in cancer therapy are also efficacious in the treatment of patients with viral disease or virus-induced immunosuppression. It should also be noted that arbitrarily dividing these immune-modulating agents into three broad categories is not intended to suggest that the mechanisms of action can be similarly segregated. Although synthetic chemical BRMs are perhaps less toxic than biological agents, most BRMs have multiple mechanisms of action directed at modulating immune responsiveness; likewise, many compounds share certain activities. The basic mechanisms described for the agents discussed in this section are, in fact, not unique to only one category.
BIOLOGICAL AGENTS Today it is generally agreed that clinical response following the use of "classic" BRMs, such as Bacillus Calmette Guerin (BCG), methanol extraction residue of BCG (MER), and Corynebacterium parvum (taxonomically the same as Propionibacterium acnes), has been inconsistent and disappointing. This has been largely attributed to the toxic consequences of administering whole organisms. In search of a product that has the beneficial and predictable effects ofBCG but lacks the toxic effects, more recent studies have concentrated on the immunoaugmenting capability of mycobacterial cell wall fractions. 23· 41 Mycobacterial fractions, as well as BCG and other microorganisms, appear to stimulate T-lymphocyte proliferation initially. This is followed by nonspecific macrophage proliferation and activation. As T lymphocytes are stimulated, a variety of soluble macrophage-directed mediators are released. Subsequently, stimulated macrophages release (1) colony-stimulating factor, which further acts to regulate macrophage proliferation and activation, and (2) interleukin-1, which further promotes lymphocyte proliferation. 23 Propionibacterium acnes Recently, a product containing a preparation of0.4 mg per ml nonviable Propionibacterium acnes (synonym: Corynebacterium parvum) suspended in 12.5 per cent ethanol in saline* was introduced into veterinary medicine *lmmunoRegulin, Immunovet, Inc., Tampa, Florida.
Table 2. COMPOUND NAME
Biological Agents Propionobacterium acnes (syn: Corynebacterium parvum) and its fractions
Lentinan
Interferons Hormones Thymosins
Chemical Agents Levamisole
Promodulint Thiazolobenzimidazole (Wy-18,251) Isoprinosine
(methisoprinol)
Tilorone and related analogs Lipoidal amines (CP-20,961) MVE
Biological Response Modifiers that Have In Vivo or In Vitro Efficacy in Viral Disease CLASS
PRIMARY MECHANISMS OF ACTION
CLINICAL APPLICATION
REFERENCES
Anaerobic coryneiform
Reticulostimulatory activity -Macrophage activation -Augments humoral and cellular im-
Adjuvant Enhances resistance to viral infec-
15, 17, 26
tion
Available for treatment of feline leukemia virus infection in cats* Mouse encephalitis (VSV) Virus-induced malignancy (adenovirus-12 and Abelsons tumor virus) Retrovirus (under investigation) Antiviral
5, 25, 35, 41
PoiYI>eptide isolated from cell-free thymic extract and as synthetic analogs
Multiple in vivo augmenting effects on cell-mediated and humoral immune
Immunodeficient states in general
3, 6, 32, 50, 56
Phenylimidazothiazole (anthelmintic agent) L-thiazolidine, 4-carboxylic acid Synthetic, noncytotoxic compound
"Jmmunorestorative" for T lymphocytes Some interferon-inducing properties Unknown
' Secretory glycoproteins
Synthetic inosine-containing complex
Synthetic compounds Synthetic aliphatic lipid
Maleic anhydride divinyl ether (copolymers)
*lmmunoregulin, Immunovet, Inc., Tampa, Florida. tLinworth Research, Dublin, Ohio,
2, 27
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Macrophage activation
0 0
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production
Potentiates CMI Accelerates maturation ofT lymphocytes Increases proliferation ofT lymphocytes Suppresses viral mRNA (antiviral) Interferon induction (?) Augments total RNA and mRNA synthesis in lymphocytes Augments humoral (IgG and IgM) and cellular immune responses Induces interferon (antiviral) in animals only Induction of interferon (antiviral)
0t"" t""
mune responses Enhances resistance to viral infection Interferon production Induction of interferon May potentiate natural killer and killer T cells T cell-oriented immunopotentiator Activates macrophages Inhibit viral replication Modulates cytocidal activities of macrophages and natural killer cells
Neutral polysaccharide extracted from the fruit bodies of a mushroom (L. edodes)
tl:l
Immunodeficient states in general
1, 41, 53
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Treatment of FeLV-negative cats with feline infectious peritonitis Vaccine adjuvant (equine herpesvirus type I)
14
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Immunodeficient states in general Antiviral
20 21, 24, 59
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Antiviral activity against both RNA and DNA viruses Topical application in rhinovirus infection Adjuvant activity for humoral re-
37, 38, 39
Antiviral activity (nephrotoxic in dogs)
9, 45
sponses
28
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and, at this writing, is available in a limited number of states. Approved for intravenous or intraperitoneal use only, the product is indicated as adjunct therapy in the treatment of clinical signs associated with viral- and bacterialinduced immunosuppression, particularly the feline leukemia virus (FeLV)related disorders, pyoderma (in dogs and cats), and equine infectious upper respiratory disease. Classically, P. acnes is an anaerobic coryneiform that has reticulostimulatory activity characterized by increased phagocytic activity, increased particle clearance, and the development of significant hepatosplenomegaly. 41 In addition, P. acnes has been shown to modulate both cellular and humoral immune responses and to have a variety of effects on hepatic microsomal enzyme activity. The spectrum of reticulostimulatory activity attributed to P. acnes is, however, far more extensive than that summarized here. The reader is referred to several recent in-depth reviews for more comprehensive discussions of this bacterial agent. 15•17•26 •36•42 With respect to clinical efficacy in infectious disease, P. acnes must be regarded as adjuvant therapy only. Preparations of P. acnes cannot be recommended as a primary treatment modality. FeLV-positive cats suffering from the immunosuppressive consequences of their infection may transiently benefit from P. acnes administration; however, efforts by the manufacturer to convert viremic cats to a virusnegative status have been discontinued. 47 The lack of definitive clinical studies and the potential toxic side effects of bacterial agents in immunomodulatory therapy have limited the widespread introduction and use of these preparations in clinical veterinary medicine. Nonetheless, compounds such as BCG and its cell wall fractions still represent an important prototype in the category of microbial immunomodulators. The potential application of these compounds in the management of infectious disease in small animals is worthy of further evaluation. Len tin an While surveying a group of Asian folk remedies that by reputation are said to be effective against cancer, investigators isolated a unique polysaccharide from Lentinus edodes, the most popular edible mushroom in Japan. This carbohydrate, called lentinan, completely regressed the solid form of sarcoma-180 transplanted into mice. 12•13 Subsequent studies have shown this compound to be effective in bacterial, viral, and parasitic infections. Lentinan appears to augment the reactivity of precursor effector cells to major lymphokines, which in turn enhance the generation of cytotoxic T lymphocytes, natural-specific killer cells, activated macrophages, and antibody. 2 Lentinan is a neutral, well-characterized nontoxic immunotherapeutic agent and as such, represents interesting potential for therapeutic regimens in patients with viral infections. There are no reports on the use of this compound in treating small animal patients with either cancer or infectious disease. Interferon Interferons are inducible secretory glycoproteins produced by virtually all nucleated animal cells and can be produced both in vivo and in vitro by eukaryotic cells in response to viral infection or other stimuli. By definition, the interferons inhibit the intracellular replication of viruses. They also in-
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
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hibit replication of many cell types and appear to modulate cytocidal activities of macrophages and natural killer cells. Although the documented biological properties attributed to interferons are extensive, only these three actions have been conclusively shown to exist in the same molecule and not in contaminants in the interferon preparations. 25 The manner in which interferon exerts antiviral action is unique. Subsequent to viral invasion, the infected cell responds by synthesizing the releasing molecules of interferon. The secreted interferon binds to receptor sites on the surface of uninfected adjacent cells to produce a group of proteins capable of inhibiting virus replication in the adjacent cell. Interestingly, the effect of interferons is not limited to the specific virus that stimulated their production. Interferon molecules interact with other cells in the body to modulate immune responses. For example, interferons can activate macrophages and increase the destructive capacity of cytotoxic T lymphocytes and natural killer cells. Subsequent to the demonstration that interferon inhibits the division of cells, an intensive research effort has been directed at elucidating the role of interferon in cancer therapy. 5 · 25 •35•41 Further discussion on the antiviral activity of interferons can be found in another section of this issue. 22
HORMONAL AGENTS A wide variety of hormones are known for their ability to alter immune responsiveness. Among the best known and most thoroughly studied are the glucocorticosteroids (GCS). Although GCS may have some value in the short-term treatment of infectious disease, 18 as BRMs, corticosteroids are immunosuppressive compounds and, as such, clinical application is generally limited to the treatment of cancer, allergies, and autoimmune disease. Two additional categories of hormone have been studied for their immunomodulating effects: prostaglandins, recognized for their immumostimulating capability, and thymic hormones, specifically thymosins, for their immunorestorative properties. Prostaglandins Prostaglandins (PGs) are local hormones that have a recognized role as modulators ofT- and B-lymphocyte function, as well as cellular interactions. However, any immunopotentiating activity that PGs have appears at this time to be minimal, because these hormones, particularly PGE 2 , are also immunosuppressive. 30 There is no documentable role for the use of these compounds in treating viral infections. Thymosins Perhaps those hormones receiving the most attention for their immunopotentiating properties are the thymic hormones, best known for their ability to enhance host resistance by accelerating T-cell maturation (Fig. 1). The role of the thymus in development and maintenance of the immune system has been known for many years. Thymosin is known to stimulate the release of pituitary neuropeptides, establishing a link between the thymus
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T CELLS SUPPRESSOR BONE MARROW
THYMUS
PERIPHERAL LYMPHOID
L yt
u
IH1gh ConcentratiOn)
T d T · - TdT'
Figure l. The proposed role of thymosin peptides in T-cell maturation. (From Low, T. L. K., and Goldstein, A. L.: Role of the thymosins as immunomodulating agents and maturation factors. In Moore, M. A. S. (ed.): Maturation Factors and Cancer. New York, Raven Press, 1982; with permission.)
and the neuroendocrine system. The thymic hormones induce maturation ofT-cell precursors in addition to promoting differentiation and proliferation of mature T cells. 57 As BRMs, the thymic hormones are usually referred to as immunorestorative agents rather than immunostimulators. Interest in the role of thymic hormones as BRMs is evidenced by the number of natural hormones (thymosin fraction 5, thymic hormone factor, thymic factor X, thymopoietic, and facteur thymique serique), and synthetic thymic peptides (for example, thymosin a 1 , thymosin {3 4 , thymopoietin, and thymulin). Thymosin a I> thymopoietin, and thymulin have been synthesized, either chemically or by genetic engineering techniques. Each of these humoral factors is produced in thymic epithelial cells, with the exception of thymic humoral factor, and each is chemically distinct, capable of selectively acting on the development of immune function. The various thymic humoral factors and their biological effects have been recently reviewed. 3 •5 •6 •32 •33 •41 •50 •56 Thymic hormones and their peptidic moieties have been found to be effective in inducing and maintaining immune functions in a variety of normal and immunodeficient animal models. Although there are few studies that support directly the use of thymic hormone therapy in viral infections, these products, as adjunct therapy in animals with virus-induced immunosuppression and other immunodeficient states, do have potential clinical application. Interleukin 2 Several studies have shown that crude, as well as purified, lymphokine preparations have the capacity to stimulate or enhance cell-mediated immune responses in vitro. One such lymphokine, originally named T-cell
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
1199
growth factor, 43 has been purified extensively and is now referred to as interleukin 2 (IL 2). The most profound effect ofiL 2 is its capacity to induce proliferation of cytotoxic T lymphocytes that are antigen-specific, as well as nonspecific killer cells. 40 It has already become apparent that these cytotoxic effector cells propagated by IL 2 may have some benefit in the treatment of cancer. 11 Furthermore, direct infusion of IL 2 during human clinical trials suggests that this purified lymphokine may be of benefit as adjunct therapy in patients with immune deficiency and complications of infectious disease. 8 •51 •53 Although interleukin 1-like activity has been identified in soluble factors isolated from leukocytes and spleen cells in dogs 10 and in vivo studies with animals suggest a positive influence of IL 2 on restoring and potentiating cell-mediated immune responses, it is still too soon to evaluate the clinical therapeutic value ofiL 2 in the management of viral infection or viral-induced immunodeficient states.
CHEMICAL AGENTS Perhaps the most significant limitation to the use of bacteria or bacterial products as BRMs has been their toxicity. Consequently, efforts have been directed at the development of chemically defined agents capable of exerting specific influences on immune responsiveness with less or no toxicity compared with biological agents discussed above. Levamisole In veterinary medicine, levamisole, a phenylimidazothiazole introduced in 1966 as an anthelmintic, has probably received more attention as a BRM than any other chemical compound. The ability of levamisole to augment the protective effect of an anti brucella vaccine against challenge with virulent brucella was first reported in 1971. 49 This report gave credibility to earlier reports in which unrelated infectious diseases were ameliorated or eliminated in animals treated with levamisole for nematode infection. Levamisole has no known direct effect on bacteria, fungi, or viruses, nor is it cytotoxic in vitro. A derivative of imidazole, levamisole acts on cyclic nucleotide phosphodiesterases to decrease the breakdown of cyclic GMP (cGMP) and increase the breakdown of cyclic AMP. The net effect of increased levels of cGMP in lymphocytes is to enhance the proliferative and secretory responses of immunocytes to mitogens. Levamisole itself is not a mitogen. 1· 23 In immunocompetent animals, levamisole has little effect on antibody production. 52 Thiazolobenzimidazoles Among the other synthetic chemical agents capable of stimulating the immune response, the thiazolobenzimidazoles have demonstrated rapid, reproducible changes in the in vitro immune response. These compounds are reportedly capable of potentiating cell-mediated immunity, increasing numbers ofT cells, and stimulating interleukin 2 (IL 2) production. 20 One compound, Wy-18,251, studied at Wyeth Laboratories, has demonstrated po-
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tential as an immunoadjuvant in an inactivated equine herpesvirus type I vaccine. 20 Indications for the routine clinical use ofWy-18,251 have not been established. Lipoidal Amines Unlike levamisole and the thiazolobenzimidazoles that have no antiviral activity, other chemical agents do have the ability to stimulate antiviral activity either by their ability to induce interferon or by their direct effect on viral DNA. It is evident that several rather simple molecules have the ability to evoke similar nonspecific responses that facilitate endogenous immune responsiveness to microorganisms or infected cells. Induction of nonspecific viral resistance by interferon is a biological property of at least two classes of chemical BRMs. One such class, the lipoidal amines, have been recognized to have antiviral activity in mice infected with RNA and DNA viruses. Although lipoidal amines do not appear to be virucidal, clinical antiviral studies with lipoidal amines have been conducted in humans before and after rhinovirus challenge, with the primary effect being a decrease in the intensity of symptoms rather than complete protection. 55 Tilorone A second compound, tilorone hydrochloride, and its analogs are orally active molecules, having antiviral activity resulting primarily from the induction of interferon. In addition, tilorone enhances immunity while suppressing cell-mediated immune responses in animals. 37•38 In rats, mice, and monkeys, tilorone and its congeners increased survival incidence, increased survival times, and actually prevented viremia following challenge with a variety of DNA and RNA viruses. 39 Isoprinosine Isoprinosine, an inosine-containing complex, has been used both experimentally and clinically in viral infections. Isoprinosine inhibits replication of both DNA and RNA viruses in tissue culture and has documented clinical efficacy in human viral disorders, including subacute sclerosing panencephalitis and cutaneous herpes. Although isoprinosine shows a degree of inhibition of replication of several viruses in vitro, rather high concentrations are required and the maximal antiviral activity does not approach that of antimetabolite antiviral compounds. Isoprinosine does have the ability to induce T-lymphocyte differentiation in a manner comparable with thymic hormones. The mechanism of action for the antiviral activity of isoprinosine is not entirely understood, yet it appears to involve specific suppression of viral messenger RNA (mRNA). 44 As an immunopotentiator, isoprinosine augments total RNA and mRNA synthesis of lymphocytes. Immunopharmacologic data support the clinical application of isoprinosine in treating immunodeficient states in general. 54
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
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Promodulin
Promodulin, * an experimental immunomodulating agent, has been used in clinical therapeutic trials for the treatment offeline leukemia virus (FeLV)negative cats with feline infectious peritonitis (FIP) virus infection. Originally evaluated for its ability to reduce or eliminate tumor burdens in FeLVinduced lymphosarcoma, Promodulin was not efficacious in treating solid tumors but did induce rapid remission of clinical signs associated with FIP, particularly anorexia, fever, and serosal effusions. Responses to treatment have been tabulated on 52 FIP-suspect cats. Twenty-one (40.4 per cent) of 52 cats were judged to be responders; 38 (59.6 per cent) of the cats treated failed to respond favorably. Most of the nonresponders were judged to have advanced disease prior to treatment. From the data reported, cats with concurrent FeLV and FIP virus infection do not benefit from therapy. 11 The duration of response following therapy was not reported. Promodulin (presumably L-thiazolidine-4-carboxylic acid) is a poorly soluble, light-sensitive crystalline compound having a limited effective shelflife once constituted. The recommended dosage in cats is 50 mg per kg given intravenously, with a maximum total dose of 200 mg per cat, once daily for 5 consecutive days. At these levels, the drug appears to be safe and is free of undesirable side effects. Transient hyperexcitability during injection was reported infrequently (less than 1.0 per cent). The ability of Promodulin therapy to induce long-term (longer than 3 months) remission or to cure FIP virus-infected cats has not been established. In the author's limited experience, remission of clinical signs lasts from 1 to 3 months before recurring. Cats receiving the full 5-day treatment protocol responded well within 2 weeks of the last injection. Cats treated with a second series of five daily injections, for exacerbation, did not respond. Nonetheless, additional clinical trials with Promodulin are warranted because the clinical manifestations of FIP are caused by a heightened immunologic response to the virus, rather than virus-induced cytotoxicity. In addition, attempts to immunize cats against FIP using autologous killed viral products actually enhance the severity of clinical disease following virulent virus challenge. 46 At this time, immunomodulatory therapy may be the most practical alternative in the management of FIP.
SUMMARY The rapid evolution-of immunopharmacology as a recognized scientific discipline dedicated to unraveling complex interrelationships between immunologic responsiveness and disease states in general supports the importance of the potential role biological response modifiers have in clinical medicine. To administer a drug, or combination of drugs, that safely, effectively, and favorably alter the course of infection, cancer, autoimmune disease, and allergy is within grasp. Although the greatest emphasis on therapeutic application of BRMs is placed on cancer, many of these *Linworth Research, Dublin, Ohio.
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immunomodulating agents have well-documented effects on the course of infectious disease. By either restoring immune responses or by enhancing the response of a normal immune system, it is conceivable that BRM therapy will someday be used routinely as adjunct therapy in the management of viral infections in companion animals.
REFERENCES l. Amery, W. K., and Horig, C.: Levamisole. In Fenichel, R., and Chirigos, M. (eds.):
2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 383-408. Aoki, T.: Lentinan. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, p. 63-77. Attallah, A. M., Yeatman, T. J., Johnson, R. P., et al.: Biological response modifiers and their promise in clinical medicine. Pharmacal. Ther., 19:435-454, 1983. August, J. R.: Feline infectious peritonitis. Vet. Clin. North Am (Small Anim. Pract.), 14:971-984, 1984. Bardana, E. J.: Recent developments in immunomodulatory therapy. J. Allergy Clin. Immunol., 75:423-436, 1985. Barrett, D. J., Ware, D. W., Ammann, A. J., et al.: Thymosin therapy in the DiGeorge syndrome. J. Pediatr., 97:66-71, 1980. Berd, D., Maquire, H. C., and Mastrangelo, M. J.: Immunopotentiation by cyclophosphamide and other cytotoxic agents. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 39-61. Bindon, C., Czerniecki, M., Ruell, P., et al.: Clearance rates and systemic effects of intravenously administered interleukin 2 (IL 2) containing preparations in human subjects. Br. J. Cancer, 47:123-133, 1983. Carrano, R. A., Iuliucci, J.D., Luce, J. K., et al.: MEV-2. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 243-260. Cerruti-Sola, S., Kristensen, F., Vandevelde, M. et al.: Interleukin 1- and 2-like activities in the dog. Vet. Immunol. Immunopathol., 6:261-271, 1984. Cheever, M. A., Greenberg, P. D., Fefer, A., et al.: Augmentation of the anti-tumor therapeutic efficacy of long term cultured T lymphocytes by in vivo administration of purified interleukin 2. J. Exp. Med., 155:968-980, 1982. Chihara, G., Maeda, Y. Y., Hamuro, J., et al.: Inhibition of mouse sarcoma 180 by polysaccharides from Lentinus edodes. Nature (Lond.), 222:687-688, 1969. Chihara, G., Hamuro, J., Maeda, Y. Y., et al.: Fractionation and purification of the polysaccharides with marked antitumor activity, especially lentinan, from Lentinus edodes. Cancer Res., 30:2776-2781, 1970. Clinical evaluation of a new immune-modulating compound on feline infectious peritonitis disease. Linworth Research, Dublin, Ohio (unpublished data). Cooper, J., Gaugas, J. M., Jimenez, J., et al.: ImmunoRegulin immunomodulator. ImmunoVet, Inc. Tampa, Florida (unpublished data). Cotter, S. M.: Feline viral neoplasia. In Greene, C. (ed.): Clinical Microbiology and Infectious Diseases of the Dog and Cat. Philadelphia, W.B. Saunders Co., 1984, pp. 490-513. Dummins, C. S.: Corynebacterium parvum and its fractions. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 163-190. Ford, R. B.: Concurrent use of corticosteroids and antimicrobial drugs in the treatment of infectious diseases in small animals. J. Am. Vet. Med. Assoc., 185:1142-1144, 1984. Glaser, M.: Augmentation of specific immune response against a syngeneic SV40-induced sarcoma in mice by depletion of suppressor T cells with cyclophosphamide. Cell. Immunol., 48:339-345, 1979. Gregory, F. J.: Thiazolobenzimidazole and thiazolobenzothiazole compounds as biological
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
42.
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response modifiers. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 21-38. Grieco, M. H., Reddy, M. M., Manvar, D., et al.: In vivo immunomodulation by Isoprinosine in patients with acquired immunodeficiency syndrome and related complexes. Ann. Intern. Med., 101:206-207, 1984. Gustafson, D. P.: Antiviral therapy. Vet. Clin. North Am. [Small Anim. Pract.], 16:1181-1189, 1986. Hadden, J. W., Delmonte, L., and Oettgen, H. F.: Mechanisms ofimmunopotentiation. In Hadden, J. W., Coffey, R. G., and Spreafico, R., (eds.): Immunopharmacology. New York, Plenum, 1977, pp. 279-313. Hadden, J.: Chemically defined immunotherapeutic drugs under development for cancer. In Serrou, B., Rosenfeld, C., and Daniels, J. (eds.): Current Concepts in Human Immunology and Cancer Immunomodulation. Amsterdam, Elsevier, 1982, pp. 439-448. Hadden, J. W., and Merriam, L. K.: Immunopharmacologic basis of immunotherapy. Clin. Physiol. Biochem., 3:111-119, 1985. Halpern, B. (ed.): Corynebacterium parvum: Applications in Experimental and Clinical Oncology. New York, Plenum Press, 1975. Hamuro, J., and Chihara, C.: Lentinan, AT-cell-oriented immunopotentiator. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 409-436. Hoffman, W. W.: Lipoidal amines. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 121-132. Humphrey, J. H.: Immunology Today, 2:V, 1981. Kuehl, F. A.: Prostaglandins in the regulation of immune and inflammatory responses. In Hadden, J. W., Coffey, R. C., and Spreafico, R.: Immunopharmacology. New York, Plenum, 1977, pp. 145-161. Loar, A. S.: The management of feline lymphosarcoma. Vet. Clin. North Am. [Small Anim. Pract.], 14:1299-1330, 1984. Low, T. K. L., and Goldstein, A. L.: Role of the thymosins as immunomodulating agents and maturation factors. In Moore, M. A. S. (ed.): Maturation Factors and Cancer. New York, Raven Press, 1982, pp. 129-152. Low, T. K. L., and Goldstein, A. L. : Thymosin, peptidic moieties, and related agents. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, p. 134-162. MacEwen, E. G.: General concepts of immunotherapy of tumors. J. Am. Anim. Hasp. Assoc., 12:363-373, 1976. MacEwen, E. G.: An immunologic approach to the treatment of cancer. Vet. Clin. North Am. [Small Anim. Pract.], 7:65-76, 1977. MacEwen, E. G.: Approaches to cancer therapy using biological response modifiers. Vet. Clin. North Am. [Small Anim. Pract.], 15:667-688, 1985. Megel, H., Rauchaudhuri, A., Goldstein, S., et al.: Paradoxical effects oftilorone hydrochloride on humoral and cell-mediated immune response in mice and rats (abstract). Fed. Proc., 32:1021, 1973. Megal, H., Rauchaudhuri, A., Goldstein, S., et al.: Tilorone: its selective effects on humoral and cell-mediated immunity. Proc. Soc. Exp. Bioi. Med., 145:513-518, 1974. Megel, H., and Gipson, J. P.: Tilorone and related analogs. In Fenichel, R., and Chirigos, M. (eds.): Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, p. 97-120. Merluzzi, V. J., and Last-Barney, K.: Potential use of human interleukin 2 as an adjunct for the therapy of neoplasia, immunodeficiency and infectious diseases. Int. J. Immunopharmacol., 7:31-39, 1985. Mihich, E., and Fefer, A. (eds.): Augmenting and immunomodulating agents. Section V. In Biological Response Modifiers. Subcommittee Report, National Cancer Institute Monograph 63. NIH Publication No. 83-2606. National Cancer Institute, Bethesda, Maryland, September, 1983, pp. 141-169. Milas, L., and Scott, M. T.: Antitumor activity of Corynebacterium parvum. Adv. Cancer Res., 26:257-306, 1978.
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43. Morgan, D. A., Ruscetti, F. W., and Gallo, R. C.; Selective in vivo growth ofT lymphocytes from normal human bone marrows. Science, 193:1007-1008, 1976. 44. Ohnishi, H., Kosuzume, H., Inaba, H., eta!.: Mechanism of host defense suppression induced by viral infection; Mode of action of inosiplex as an antiviral agent. Infect. Immun., 38:243-250, 1982. 45. Pearson, J. W., Chirigos, M. A., Chaparas, S. D., eta!.: Combined drug and immunostimulation therapy against a syngeneic murine leukemia. J. Nat!. Cancer Inst., 52:463-468, 1974. 46. Pedersen, N. C. ,and Black, J. W.; Attempted immunization of cats against feline infectious peritonitis using either anti virulent live virus or sublethal amounts of virulent virus. Am. J. Vet. Res., 44:229-234, 1983. 47. Connell, J. A.: Personal communication. 48. Ray, W. J., and Cooper, J. A.; Chronic deep pyoderma in a German shepherd. Canine Pract., 11:35-36, 1984. 49. Renoux, G., and Renoux, M.; Effet immunostimulant d'un imidothiazole das !'immunisation des souris contre !'infection par Brucella abortus. C.R. Acad. Sci. [D] (Paris), 272:349-350, 1971. 50. Schulof, R. S., Low, T. L. K., Thurman, G. B., eta!.: Thymosins and other hormones of the thymus gland. In Sell, K. W., and Miller, M. V. (eds.); The Lymphocyte. New York, Alan R. Liss, 1981, pp. 191-215. 51. Speigel, J.P., Rook, R. H., Djeu, G. V., eta!.: Interleukin 2 therapy in infectious disease; Rationale and prospects. Infection, 12:298-301, 1984. 52. Symoens, J.; Levamisole, an antianergic chemotherapeutic agent: An overview. In Chirigos, M. A. (ed.): Control of Neoplasia by Modulation of the Immune System. New York, Raven Press, 1977, pp. 1-24. 53. Symoens, J., De Cree, J., Van Bever, W. F. M., eta!.: Levamisole. In Goldberg, M. E. (ed.): Pharmacological and Biochemical Properties of Drug Substances. Vol. 2. Washington, D. C., American Pharmaceutical Association Academy of Pharmaceutical Sciences, 1979, pp. 409-464. 54. Tsang, K. W., Fudenberg, H. H., Hoehler, F. K., eta!.; Immunostimulating compounds. In Fenichel, R., and Chirigos, M. (eds.); Immune Modulation Agents and Their Mechanisms. New York, Marcel Dekker, Inc., 1984, pp. 79-95. 55. Waldman, R. H., and Ganguly, R.: Effect ofCP-20,961, an interferon inducer, on upper respiratory tract infections due to rhinovirus type 21 in volunteers. J. Infect. Dis., 138:531-535, 1978. 56. Wara, D. W., Barrett, D. J., Ammann, A. J., eta!.: In vitro and in vivo enhancement of mixed lymphocyte culture reactivity by thymosin in patients with primary immunodeficiency disease. Ann. N.Y. Acad. Sci., 332:128-134, 1980. 57. Wara, D. W.; Thymic hormones in primary immunodeficiency. Clin. Immunol. Allergy, 3:169-175, 1983. 58. Winters, W. D., and Harris, S. D.: Increased survival and interferon induction by BCG cell wall immunotherapy in pet dogs with malignant mammary tumors. In Proceedings of the 73rd Annual Meeting of the American Association of Cancer Research, 23:244, 1982. (abstract). 59. Wybran, J., Famaey, J. P., Gortz, R., et a!.: Inosiplex (Isoprinosine); A review of its immunological and clinical effects on disease. Adv. Pharmacal. Ther. II, 6:123-131, 1982. Department of Companion Animal and Special Species Medicine School of Veterinary Medicine North Carolina State University 4700 Hillsborough Street Raleigh, North Carolina 27606
0195-5616/86
$0.00+ .20
Biological Response Modifiers in the Management of Viral Infection Richard B. Ford, D.V.M., M.S.*
The year 1908 seems to have been a hallmark year in the history of therapeutic immunomodulation, perhaps best referred to today as immunopharmacology. In that year, Calmette and Guerin, while working at the Pasteur Institute, developed an effective vaccine against human tuberculosis using an attenuated strain of Mycobacterium bovis, the Bacillus Calmette Guerin (BCG). 41 Also in that year in Holland, Paul Ehrlich suggested that the body contained intrinsic "protective devices" capable of causing aberrant cells to remain latent for decades rather than to develop into cancer. 29 This early concept on the theory of immune surveillance has since been extensively elaborated on and is probably one reason that biological response modifiers (BRMs) are receiving attention today. It is interesing to note, however, that only during the past 40 years has our knowledge of immunologic responses progressed from the somewhat rudimentary description of intrinsic host defense mechanisms to a more comprehensive understanding of cellular and molecular processes involved in the immune system. It has only been during the past 20 years that investigators and clinicians have gained an appreciation of the role certain agents have in augmenting immune responsiveness. As several new waves of research attempt to narrow the gap between experimental observations with BRMs and clinical therapeutic trials, the concept of a pharmacology that can modify and even control immunologic function continues to emerge. Although in its infancy in human and veterinary medicine, the potential role of BRM therapy in managing cancer, autoimmune disease, infectious diseases, and allergies continues to generate considerable interest. Clearly, the concept of altering a disease process in a given patient by administering chemical or biological agents that enhance immune responsiveness has surpassed theoretical acceptance. Human clinical trials, particularly in cancer patients, involving BRMs are widespread today. In contrast, relatively little information is available on the clinical applications of immunomodulating *Diplomate, American College of Veterinary Internal Medicine; Associate Professor, Department of Companion Animal and Special Species Medicine, North Carolina State University School of Veterinary Medicine, Raleigh, North Carolina
Veterinary Clinics of North America: Small Animal Practice-Vol. 16, No. 6, November 1986
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agents in veterinary medicine. 4 •14•15•16•31 •34•35•36•48 •58 Despite the fact that BRMs are not in widespread clinical use in veterinary medicine, their existence and limited availability are gradually becoming known to practitioners, as well as clientele. With time, information about these products, as well as their therapeutic claims, will undoubtedly stimulate interest among owners of pets that might benefit from such therapy. Compared with the extensive volume of literature addressing the role of BRMs in the treatment of cancer, the number of reports that address the role of these agents in the treatment of patients with infectious disease, particularly patients with viral infections, is relatively small. Nonetheless, such products do exist and are currently being marketed in veterinary medicine. Unfortunately, there is far too little information available on which to judge clinical efficacy of the products currently marketed in the United States. The intensive investigations directed at finding a cure for acquired immunodeficiency syndrome (AIDS) will undoubtedly result in the development of several immunomodulating agents, as well as antiviral compounds, during the next 3 to 4 years. As immunopharmacology continues to emerge as a recognized clinical discipline, many new and more effective agents will undoubtedly be introduced into human and veterinary medicine. The burden of responsibility ultimately falls upon the clinician to understand the clinical indications of these agents, as well as to critically assess therapeutic efficacy and safety of immunomodulatory therapy. In the treatment of cancer, BRMs have been introduced on the premise that cancers develop within the framework of immunodeficiency or at least of defective immune surveillance. In addition, tumors are antigenic and, if reacted to by a stimulated immune system, would either regress or not recur following removal. This, in addition to the fact that cancer is generally associated with progressive immunodeficiency and is compounded by immunosuppression associated with other therapies explains the current level of interest and research into immunomodulatory therapy. 33 Biological response modifiers do have a role in the management of viral infections, particularly in infections caused by viruses capable of inducing immunosuppression. However, the precise role of these agents is less clearly understood and has not been investigated to the same extent as BRM therapy in cancer. The intent of this article, therefore, is to review the BRMs that have reported efficacy in the treatment of viral infections and to summarize their mechanisms of action and potential clinical application. The discussion that follows focuses only on BRMs currently under investigation and on those that have demonstrated some potential, either in vitro or in vivo, for augmenting immune responsiveness in animals with viral infections. BRMs DEFINED Also referred to as immunomodulators, immunoaugmentors, immunoadjuvants, immunorestoratives, immunopotentiators, and immunostimulators, the term "biological response modifier" refers to individual agents, either biological or synthetic, that are capable of eliciting specific and/or
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
Table l.
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Categories of Biological Response Modifiers and Representative Agents BIOLOGICAL RESPONSE MODIFIERS USED IN IMMUNOTHERAPY
"Prohost" Agents Lentinan Tilorone Lipoidal amines MVE (maleic anhydride divinyl ether)
Immunorestoratives Passive immunization with gamma globulin Plasma infusion Immune serum globulin Interferon Thymic hormones (thymosins) Marrow, thymus, fetal tissue transplantation
Noncytotoxic Immunosuppressive Agents Cyclosporins Apheresis*
Immunostimulants Dialyzable leukocyte extracts Transfer factor Levamisole BCG Propionibacterium acnes (syn: Corynebacterium parvum) Tuftsin Bestatin Isoprinosine
*In contrast to plasmapheresis, apheresis refers to the process of withdrawing a patient's blood, separating, and removing specific constituents, such as plasma, huffy coat, or platelets, prior to reinfusing the desired components.
nonspecific effects on immune responsiveness. Clearly, the role of BRMs in "prohost" therapy has been emphasized. "Prohost" therapy refers to those agents that augment the cellular immune response and may ultimately lead to an improvement in the overall health of the patient. Such agents may be further subcategorized into those that facilitate a normal immune responseimmunorestoratives-and those that stimulate the immune response-immunostimulants. Another group ofBRMs is capable of inducing noncytotoxic immunosuppression. 7 •19 Such agents are currently being studied for their role in the treatment of autoimmune disease and in tissue rejection inhibition following organ transplantation (Table 1). Some degree of immune impairment is associated with most disease states, particularly systemic disorders such as cancer, autoimmune disease, and infectious diseases. Therefore, the assumption that patients affected with a systemic disease would benefit from therapy that either restores immune responsiveness to its normal "resting" state or that augments the normal immune system seems logical. However, clinical trials with BRMs in humans and animals do not consistently support this premise. The assays used to establish possible efficacy of a given compound as a BRM are largely based on in vitro test systems and, as such, have generated greater expectations than can be produced in clinical practice. Perhaps this is attributable to the fact that clinical therapeutic trials are commonly carried out on patients with overwhelming infections or during the terminal stages of a disease. Yet the clinician, as well as the investigator, must contend with numerous other variables in the living animal, including the host-virus relationship, the impact of other therapies on immune responsiveness, and the function of immunocompetent cells.
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BRMs USED IN THE TREATMENT OF VIRAL INFECTIONS A universally accepted system for classifYing BRMs has not been developed. Generally, most discussions on the role of BRMs in cancer management and therapy either do not attempt to categorize individual agents or segregate them by chemical classification or mechanism of action. For purposes of this discussion, BRMs that have at least potential efficacy in the treatment of viral infections are segregated into three broad categories: biological agents, hormonal agents, and chemical agents. These categories and the agents within each category are summarized in Table 2. Although all of the compounds listed in Table 2 have documented antitumor properties, this by no means suggests that all BRMs used in cancer therapy are also efficacious in the treatment of patients with viral disease or virus-induced immunosuppression. It should also be noted that arbitrarily dividing these immune-modulating agents into three broad categories is not intended to suggest that the mechanisms of action can be similarly segregated. Although synthetic chemical BRMs are perhaps less toxic than biological agents, most BRMs have multiple mechanisms of action directed at modulating immune responsiveness; likewise, many compounds share certain activities. The basic mechanisms described for the agents discussed in this section are, in fact, not unique to only one category.
BIOLOGICAL AGENTS Today it is generally agreed that clinical response following the use of "classic" BRMs, such as Bacillus Calmette Guerin (BCG), methanol extraction residue of BCG (MER), and Corynebacterium parvum (taxonomically the same as Propionibacterium acnes), has been inconsistent and disappointing. This has been largely attributed to the toxic consequences of administering whole organisms. In search of a product that has the beneficial and predictable effects ofBCG but lacks the toxic effects, more recent studies have concentrated on the immunoaugmenting capability of mycobacterial cell wall fractions. 23· 41 Mycobacterial fractions, as well as BCG and other microorganisms, appear to stimulate T-lymphocyte proliferation initially. This is followed by nonspecific macrophage proliferation and activation. As T lymphocytes are stimulated, a variety of soluble macrophage-directed mediators are released. Subsequently, stimulated macrophages release (1) colony-stimulating factor, which further acts to regulate macrophage proliferation and activation, and (2) interleukin-1, which further promotes lymphocyte proliferation. 23 Propionibacterium acnes Recently, a product containing a preparation of0.4 mg per ml nonviable Propionibacterium acnes (synonym: Corynebacterium parvum) suspended in 12.5 per cent ethanol in saline* was introduced into veterinary medicine *lmmunoRegulin, Immunovet, Inc., Tampa, Florida.
Table 2. COMPOUND NAME
Biological Agents Propionobacterium acnes (syn: Corynebacterium parvum) and its fractions
Lentinan
Interferons Hormones Thymosins
Chemical Agents Levamisole
Promodulint Thiazolobenzimidazole (Wy-18,251) Isoprinosine
(methisoprinol)
Tilorone and related analogs Lipoidal amines (CP-20,961) MVE
Biological Response Modifiers that Have In Vivo or In Vitro Efficacy in Viral Disease CLASS
PRIMARY MECHANISMS OF ACTION
CLINICAL APPLICATION
REFERENCES
Anaerobic coryneiform
Reticulostimulatory activity -Macrophage activation -Augments humoral and cellular im-
Adjuvant Enhances resistance to viral infec-
15, 17, 26
tion
Available for treatment of feline leukemia virus infection in cats* Mouse encephalitis (VSV) Virus-induced malignancy (adenovirus-12 and Abelsons tumor virus) Retrovirus (under investigation) Antiviral
5, 25, 35, 41
PoiYI>eptide isolated from cell-free thymic extract and as synthetic analogs
Multiple in vivo augmenting effects on cell-mediated and humoral immune
Immunodeficient states in general
3, 6, 32, 50, 56
Phenylimidazothiazole (anthelmintic agent) L-thiazolidine, 4-carboxylic acid Synthetic, noncytotoxic compound
"Jmmunorestorative" for T lymphocytes Some interferon-inducing properties Unknown
' Secretory glycoproteins
Synthetic inosine-containing complex
Synthetic compounds Synthetic aliphatic lipid
Maleic anhydride divinyl ether (copolymers)
*lmmunoregulin, Immunovet, Inc., Tampa, Florida. tLinworth Research, Dublin, Ohio,
2, 27
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responses, as well as on interferon
Macrophage activation
0 0
~ t"l
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production
Potentiates CMI Accelerates maturation ofT lymphocytes Increases proliferation ofT lymphocytes Suppresses viral mRNA (antiviral) Interferon induction (?) Augments total RNA and mRNA synthesis in lymphocytes Augments humoral (IgG and IgM) and cellular immune responses Induces interferon (antiviral) in animals only Induction of interferon (antiviral)
0t"" t""
mune responses Enhances resistance to viral infection Interferon production Induction of interferon May potentiate natural killer and killer T cells T cell-oriented immunopotentiator Activates macrophages Inhibit viral replication Modulates cytocidal activities of macrophages and natural killer cells
Neutral polysaccharide extracted from the fruit bodies of a mushroom (L. edodes)
tl:l
Immunodeficient states in general
1, 41, 53
~
Treatment of FeLV-negative cats with feline infectious peritonitis Vaccine adjuvant (equine herpesvirus type I)
14
0
Immunodeficient states in general Antiviral
20 21, 24, 59
>
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t"l
:::: t"l
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Antiviral activity against both RNA and DNA viruses Topical application in rhinovirus infection Adjuvant activity for humoral re-
37, 38, 39
Antiviral activity (nephrotoxic in dogs)
9, 45
sponses
28
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1196
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and, at this writing, is available in a limited number of states. Approved for intravenous or intraperitoneal use only, the product is indicated as adjunct therapy in the treatment of clinical signs associated with viral- and bacterialinduced immunosuppression, particularly the feline leukemia virus (FeLV)related disorders, pyoderma (in dogs and cats), and equine infectious upper respiratory disease. Classically, P. acnes is an anaerobic coryneiform that has reticulostimulatory activity characterized by increased phagocytic activity, increased particle clearance, and the development of significant hepatosplenomegaly. 41 In addition, P. acnes has been shown to modulate both cellular and humoral immune responses and to have a variety of effects on hepatic microsomal enzyme activity. The spectrum of reticulostimulatory activity attributed to P. acnes is, however, far more extensive than that summarized here. The reader is referred to several recent in-depth reviews for more comprehensive discussions of this bacterial agent. 15•17•26 •36•42 With respect to clinical efficacy in infectious disease, P. acnes must be regarded as adjuvant therapy only. Preparations of P. acnes cannot be recommended as a primary treatment modality. FeLV-positive cats suffering from the immunosuppressive consequences of their infection may transiently benefit from P. acnes administration; however, efforts by the manufacturer to convert viremic cats to a virusnegative status have been discontinued. 47 The lack of definitive clinical studies and the potential toxic side effects of bacterial agents in immunomodulatory therapy have limited the widespread introduction and use of these preparations in clinical veterinary medicine. Nonetheless, compounds such as BCG and its cell wall fractions still represent an important prototype in the category of microbial immunomodulators. The potential application of these compounds in the management of infectious disease in small animals is worthy of further evaluation. Len tin an While surveying a group of Asian folk remedies that by reputation are said to be effective against cancer, investigators isolated a unique polysaccharide from Lentinus edodes, the most popular edible mushroom in Japan. This carbohydrate, called lentinan, completely regressed the solid form of sarcoma-180 transplanted into mice. 12•13 Subsequent studies have shown this compound to be effective in bacterial, viral, and parasitic infections. Lentinan appears to augment the reactivity of precursor effector cells to major lymphokines, which in turn enhance the generation of cytotoxic T lymphocytes, natural-specific killer cells, activated macrophages, and antibody. 2 Lentinan is a neutral, well-characterized nontoxic immunotherapeutic agent and as such, represents interesting potential for therapeutic regimens in patients with viral infections. There are no reports on the use of this compound in treating small animal patients with either cancer or infectious disease. Interferon Interferons are inducible secretory glycoproteins produced by virtually all nucleated animal cells and can be produced both in vivo and in vitro by eukaryotic cells in response to viral infection or other stimuli. By definition, the interferons inhibit the intracellular replication of viruses. They also in-
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
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hibit replication of many cell types and appear to modulate cytocidal activities of macrophages and natural killer cells. Although the documented biological properties attributed to interferons are extensive, only these three actions have been conclusively shown to exist in the same molecule and not in contaminants in the interferon preparations. 25 The manner in which interferon exerts antiviral action is unique. Subsequent to viral invasion, the infected cell responds by synthesizing the releasing molecules of interferon. The secreted interferon binds to receptor sites on the surface of uninfected adjacent cells to produce a group of proteins capable of inhibiting virus replication in the adjacent cell. Interestingly, the effect of interferons is not limited to the specific virus that stimulated their production. Interferon molecules interact with other cells in the body to modulate immune responses. For example, interferons can activate macrophages and increase the destructive capacity of cytotoxic T lymphocytes and natural killer cells. Subsequent to the demonstration that interferon inhibits the division of cells, an intensive research effort has been directed at elucidating the role of interferon in cancer therapy. 5 · 25 •35•41 Further discussion on the antiviral activity of interferons can be found in another section of this issue. 22
HORMONAL AGENTS A wide variety of hormones are known for their ability to alter immune responsiveness. Among the best known and most thoroughly studied are the glucocorticosteroids (GCS). Although GCS may have some value in the short-term treatment of infectious disease, 18 as BRMs, corticosteroids are immunosuppressive compounds and, as such, clinical application is generally limited to the treatment of cancer, allergies, and autoimmune disease. Two additional categories of hormone have been studied for their immunomodulating effects: prostaglandins, recognized for their immumostimulating capability, and thymic hormones, specifically thymosins, for their immunorestorative properties. Prostaglandins Prostaglandins (PGs) are local hormones that have a recognized role as modulators ofT- and B-lymphocyte function, as well as cellular interactions. However, any immunopotentiating activity that PGs have appears at this time to be minimal, because these hormones, particularly PGE 2 , are also immunosuppressive. 30 There is no documentable role for the use of these compounds in treating viral infections. Thymosins Perhaps those hormones receiving the most attention for their immunopotentiating properties are the thymic hormones, best known for their ability to enhance host resistance by accelerating T-cell maturation (Fig. 1). The role of the thymus in development and maintenance of the immune system has been known for many years. Thymosin is known to stimulate the release of pituitary neuropeptides, establishing a link between the thymus
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T CELLS SUPPRESSOR BONE MARROW
THYMUS
PERIPHERAL LYMPHOID
L yt
u
IH1gh ConcentratiOn)
T d T · - TdT'
Figure l. The proposed role of thymosin peptides in T-cell maturation. (From Low, T. L. K., and Goldstein, A. L.: Role of the thymosins as immunomodulating agents and maturation factors. In Moore, M. A. S. (ed.): Maturation Factors and Cancer. New York, Raven Press, 1982; with permission.)
and the neuroendocrine system. The thymic hormones induce maturation ofT-cell precursors in addition to promoting differentiation and proliferation of mature T cells. 57 As BRMs, the thymic hormones are usually referred to as immunorestorative agents rather than immunostimulators. Interest in the role of thymic hormones as BRMs is evidenced by the number of natural hormones (thymosin fraction 5, thymic hormone factor, thymic factor X, thymopoietic, and facteur thymique serique), and synthetic thymic peptides (for example, thymosin a 1 , thymosin {3 4 , thymopoietin, and thymulin). Thymosin a I> thymopoietin, and thymulin have been synthesized, either chemically or by genetic engineering techniques. Each of these humoral factors is produced in thymic epithelial cells, with the exception of thymic humoral factor, and each is chemically distinct, capable of selectively acting on the development of immune function. The various thymic humoral factors and their biological effects have been recently reviewed. 3 •5 •6 •32 •33 •41 •50 •56 Thymic hormones and their peptidic moieties have been found to be effective in inducing and maintaining immune functions in a variety of normal and immunodeficient animal models. Although there are few studies that support directly the use of thymic hormone therapy in viral infections, these products, as adjunct therapy in animals with virus-induced immunosuppression and other immunodeficient states, do have potential clinical application. Interleukin 2 Several studies have shown that crude, as well as purified, lymphokine preparations have the capacity to stimulate or enhance cell-mediated immune responses in vitro. One such lymphokine, originally named T-cell
BIOLOGICAL RESPONSE MODIFIERS IN THE MANAGEMENT OF VIRAL INFECTION
1199
growth factor, 43 has been purified extensively and is now referred to as interleukin 2 (IL 2). The most profound effect ofiL 2 is its capacity to induce proliferation of cytotoxic T lymphocytes that are antigen-specific, as well as nonspecific killer cells. 40 It has already become apparent that these cytotoxic effector cells propagated by IL 2 may have some benefit in the treatment of cancer. 11 Furthermore, direct infusion of IL 2 during human clinical trials suggests that this purified lymphokine may be of benefit as adjunct therapy in patients with immune deficiency and complications of infectious disease. 8 •51 •53 Although interleukin 1-like activity has been identified in soluble factors isolated from leukocytes and spleen cells in dogs 10 and in vivo studies with animals suggest a positive influence of IL 2 on restoring and potentiating cell-mediated immune responses, it is still too soon to evaluate the clinical therapeutic value ofiL 2 in the management of viral infection or viral-induced immunodeficient states.
CHEMICAL AGENTS Perhaps the most significant limitation to the use of bacteria or bacterial products as BRMs has been their toxicity. Consequently, efforts have been directed at the development of chemically defined agents capable of exerting specific influences on immune responsiveness with less or no toxicity compared with biological agents discussed above. Levamisole In veterinary medicine, levamisole, a phenylimidazothiazole introduced in 1966 as an anthelmintic, has probably received more attention as a BRM than any other chemical compound. The ability of levamisole to augment the protective effect of an anti brucella vaccine against challenge with virulent brucella was first reported in 1971. 49 This report gave credibility to earlier reports in which unrelated infectious diseases were ameliorated or eliminated in animals treated with levamisole for nematode infection. Levamisole has no known direct effect on bacteria, fungi, or viruses, nor is it cytotoxic in vitro. A derivative of imidazole, levamisole acts on cyclic nucleotide phosphodiesterases to decrease the breakdown of cyclic GMP (cGMP) and increase the breakdown of cyclic AMP. The net effect of increased levels of cGMP in lymphocytes is to enhance the proliferative and secretory responses of immunocytes to mitogens. Levamisole itself is not a mitogen. 1· 23 In immunocompetent animals, levamisole has little effect on antibody production. 52 Thiazolobenzimidazoles Among the other synthetic chemical agents capable of stimulating the immune response, the thiazolobenzimidazoles have demonstrated rapid, reproducible changes in the in vitro immune response. These compounds are reportedly capable of potentiating cell-mediated immunity, increasing numbers ofT cells, and stimulating interleukin 2 (IL 2) production. 20 One compound, Wy-18,251, studied at Wyeth Laboratories, has demonstrated po-
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tential as an immunoadjuvant in an inactivated equine herpesvirus type I vaccine. 20 Indications for the routine clinical use ofWy-18,251 have not been established. Lipoidal Amines Unlike levamisole and the thiazolobenzimidazoles that have no antiviral activity, other chemical agents do have the ability to stimulate antiviral activity either by their ability to induce interferon or by their direct effect on viral DNA. It is evident that several rather simple molecules have the ability to evoke similar nonspecific responses that facilitate endogenous immune responsiveness to microorganisms or infected cells. Induction of nonspecific viral resistance by interferon is a biological property of at least two classes of chemical BRMs. One such class, the lipoidal amines, have been recognized to have antiviral activity in mice infected with RNA and DNA viruses. Although lipoidal amines do not appear to be virucidal, clinical antiviral studies with lipoidal amines have been conducted in humans before and after rhinovirus challenge, with the primary effect being a decrease in the intensity of symptoms rather than complete protection. 55 Tilorone A second compound, tilorone hydrochloride, and its analogs are orally active molecules, having antiviral activity resulting primarily from the induction of interferon. In addition, tilorone enhances immunity while suppressing cell-mediated immune responses in animals. 37•38 In rats, mice, and monkeys, tilorone and its congeners increased survival incidence, increased survival times, and actually prevented viremia following challenge with a variety of DNA and RNA viruses. 39 Isoprinosine Isoprinosine, an inosine-containing complex, has been used both experimentally and clinically in viral infections. Isoprinosine inhibits replication of both DNA and RNA viruses in tissue culture and has documented clinical efficacy in human viral disorders, including subacute sclerosing panencephalitis and cutaneous herpes. Although isoprinosine shows a degree of inhibition of replication of several viruses in vitro, rather high concentrations are required and the maximal antiviral activity does not approach that of antimetabolite antiviral compounds. Isoprinosine does have the ability to induce T-lymphocyte differentiation in a manner comparable with thymic hormones. The mechanism of action for the antiviral activity of isoprinosine is not entirely understood, yet it appears to involve specific suppression of viral messenger RNA (mRNA). 44 As an immunopotentiator, isoprinosine augments total RNA and mRNA synthesis of lymphocytes. Immunopharmacologic data support the clinical application of isoprinosine in treating immunodeficient states in general. 54
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Promodulin
Promodulin, * an experimental immunomodulating agent, has been used in clinical therapeutic trials for the treatment offeline leukemia virus (FeLV)negative cats with feline infectious peritonitis (FIP) virus infection. Originally evaluated for its ability to reduce or eliminate tumor burdens in FeLVinduced lymphosarcoma, Promodulin was not efficacious in treating solid tumors but did induce rapid remission of clinical signs associated with FIP, particularly anorexia, fever, and serosal effusions. Responses to treatment have been tabulated on 52 FIP-suspect cats. Twenty-one (40.4 per cent) of 52 cats were judged to be responders; 38 (59.6 per cent) of the cats treated failed to respond favorably. Most of the nonresponders were judged to have advanced disease prior to treatment. From the data reported, cats with concurrent FeLV and FIP virus infection do not benefit from therapy. 11 The duration of response following therapy was not reported. Promodulin (presumably L-thiazolidine-4-carboxylic acid) is a poorly soluble, light-sensitive crystalline compound having a limited effective shelflife once constituted. The recommended dosage in cats is 50 mg per kg given intravenously, with a maximum total dose of 200 mg per cat, once daily for 5 consecutive days. At these levels, the drug appears to be safe and is free of undesirable side effects. Transient hyperexcitability during injection was reported infrequently (less than 1.0 per cent). The ability of Promodulin therapy to induce long-term (longer than 3 months) remission or to cure FIP virus-infected cats has not been established. In the author's limited experience, remission of clinical signs lasts from 1 to 3 months before recurring. Cats receiving the full 5-day treatment protocol responded well within 2 weeks of the last injection. Cats treated with a second series of five daily injections, for exacerbation, did not respond. Nonetheless, additional clinical trials with Promodulin are warranted because the clinical manifestations of FIP are caused by a heightened immunologic response to the virus, rather than virus-induced cytotoxicity. In addition, attempts to immunize cats against FIP using autologous killed viral products actually enhance the severity of clinical disease following virulent virus challenge. 46 At this time, immunomodulatory therapy may be the most practical alternative in the management of FIP.
SUMMARY The rapid evolution-of immunopharmacology as a recognized scientific discipline dedicated to unraveling complex interrelationships between immunologic responsiveness and disease states in general supports the importance of the potential role biological response modifiers have in clinical medicine. To administer a drug, or combination of drugs, that safely, effectively, and favorably alter the course of infection, cancer, autoimmune disease, and allergy is within grasp. Although the greatest emphasis on therapeutic application of BRMs is placed on cancer, many of these *Linworth Research, Dublin, Ohio.
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immunomodulating agents have well-documented effects on the course of infectious disease. By either restoring immune responses or by enhancing the response of a normal immune system, it is conceivable that BRM therapy will someday be used routinely as adjunct therapy in the management of viral infections in companion animals.
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