- Email: [email protected]
TRIAL DESIGN FOR VACCINES
3
TRIAL DESIGN FOR VACCINES
PART A. Clinical Development of New Vaccines: Phase 1 and 2 Trials w . RIPLEY B A L L O U Immunology and Infectious Diseases, Clinical Development, Medimmune, Inc., Gaithersburg, Maryland 20878
I.
INTRODUCTION
The scientific basis for the development of new vaccines has accelerated greatly over the past 20 years. Major advances in the understanding of the pathophysiology of infectious diseases and a wealth of revolutionary technologies are expected to greatly enhance the feasibility of immunization against diseases for which vaccines do not currently exist [1]. Although relatively few experimental vaccines will make the transition from preclinical studies into trials involving human subjects, careful planning for clinical development activities can ensure that the most promising candidates are evaluated as efficiently as possible. The clinical development of a new vaccine begins with a plan. The clinical development plan serves as a road map and time line and should lay out, in synopsis form, a series of clinical studies designed to provide the data needed for licensure. The plan should encompass phase 1 through phase 3 activities and identify issues that willneed to be addressed in phase 4. The plan should begin with an explicit description of the proposed indications for which licensure will be pursued. The studies required to obtain the data supporting these indications should be described. Critical go-no-go milestones and alternative strategies should be identified. The majority of the clinical studies described in the development plan can be expected to occur in the phase 1 and phase 2 programs. The phase 1 and phase 2 clinical programs should be expected to overlap with preclinical development. This will require an active dialogue between clinicians and research scientists. The preclinical phase provides the data that The Vaccine Book
Copyright 2003, Elsevier Science (USA). All rights reserved.
85
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BALLOU
support the rationale for antigen selection, formulation, delivery route, and dose range to be employed in the clinic. Other preclinical objectives include the identification of an appropriate formulation capable of delivering the predicted dose range, an estimation of the potential need for booster doses, and an adequate preclinical safety database (usually including the site of injection toxicology studies that equal or exceed the doses and number of injections intended for human subjects). Additionally, well-characterized, high-throughput laboratory methods for assessing immunogenicity must be developed to support the clinical program. Clear, succinct, and internally consistent protocols are essential for the success of a clinical development program. The clinical protocol is the definitive reference for study conduct [2]. Protocols should have a limited number of objectives (generally less than three), and these should be ranked as to their priority. The investigators must force themselves to stipulate the questions being asked, the data necessary to answer those questions, and methods that must be employed to collect those data. Integral to this process is the careful design of case report forms (CRFs), which will contain the entire set of data available for analysis at study completion. If the data are not in the CRF, they cannot be analyzed. Similarly, if the data are not going to be analyzed, they should not be collected in the CRE Common mistakes among inexperienced investigators include poorly articulated objectives, too many objectives, and collection of data not required to achieve the objective. The tendency to collect data peripheral to the two or three most important objectives must be resisted. The practice of writing protocols collaboratively should be avoided as it tends to result in redundancies and inconsistencies that may lead to confusion among investigators. Careful selection of investigators having previous experience with vaccine studies and access to suitable study populations and recruitment of a sufficient number of sites to insure rapid study enrollment are important keys to success. Investigator meetings are extremely valuable for insuring that each participating site is fully trained and understands the protocol-required procedures. The mechanics of data collection must be understood in great detail. Distribution and management of the investigational agent at the study sites require careful planning. Management of laboratory specimens for determination of vaccine safety and for measurement of immune responses must be coordinated, and systems must be established for reporting and analysis of study data. The study investigators must be thoroughly familiar with the principles of good clinical practice (GCP) as described in the International Conference on Harmonization (ICH) Guidelines for Good Clinical Practice (Section E6) and Clinical Safety Data Management (Section E2A) [3]. These documents detail the responsibilities that the investigator, sponsor, and investigational review board assume when conducting studies involving human subjects, including the commitment of study investigators to conduct the study strictly according to the protocol. The ICH documents also provide essential information about what must be included in the clinical trial protocol, informed consent document, and investigator's brochure. Vaccine development has a long tradition of investigator-driven research. Beginning with William Jenner and continuing until relatively recently, vaccinologists developed their own vaccines and tested them on themselves and
3.TRIAL DESIGN FORVACCINESmA. Clinical Development of NewVaccines
87
their colleagues. Although current vaccine development still relies heavily on individuals who champion a vaccine strategy, there is now a far greater emphasis on understanding vaccine safety and therefore much greater regulatory oversight of all aspects of vaccine development. Investigators who wish to develop vaccines for the future must be committed to a strong multidisciplinary team approach that includes research and development, clinical development, investigational drug safety, regulatory affairs, biostatistics, and data management and that maintains a dialogue with governmental regulatory agencies. They must also insure that adequate organization and financial and personnel resources are available to drive the program through to completion.
II.
PHASE I:A PRIMARY FOCUS ON SAFETY
The phase i program is the period of clinical development during which a vaccine is introduced for the first time, usually into a study population consisting of healthy adult volunteers who do not have preexisting acquired immunity to the disease that the vaccine is designed to prevent. The choice of a naive population permits a detailed safety evaluation in the absence of preexisting immunity to the vaccine antigen and maximizes the interpretability of elicited immune responses. The evaluation of safety requires the establishment of a baseline prior to receiving the first dose of vaccine and the measurement of changes through serial clinical and laboratory evaluation after immunization. The early clinical development of a new vaccine differs somewhat from that of small molecules and other biologicals where pharmacokinetic and pharmacodynamic studies are essential components of the development program. Because vaccine antigens are administered in discrete, widely spaced, and relatively small amounts, the distribution, accumulation, and elimination of the vaccine antigen are rarely described. In contrast, the local and general toxicity of the formulation and the immune responses generated by the vaccine are studied in great detail. Phase 1 studies generally enroll fewer than 50 subjects and, thus, only permit a first-cut evaluation of vaccine safety using a systematic process of collecting adverse events (AEs). The number of volunteers exposed to the new vaccine will be expanded during the phase 2 program, in which several hundred volunteers can be enrolled per study. The safety data collection process is cumulative; summaries should be updated regularly and described in the investigator's brochure. ICH guideline E2A provides clear definitions for AEs and serious adverse events (SAEs). The emphasis should be on detecting important dose-limiting toxicities, recognizing that only the most frequent AEs and SAEs are likely to be detected in phase 1. An important safety readout in phase 1 vaccine trials is reactogenicity, a measure of the local and general inflammatory responses to one or more components of the formulation. Although there is frequently no direct link between vaccine reactogenicity and immunogenicity, inflammatory processes at the cellular level clearly play a crucial role in the induction of immune responses. Reactogenicity may vary substantially among individuals who
88
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develop equivalent immune responses to a vaccine. Reactogenicity is characterized by collecting solicited adverse events. Solicited AEs include local reactions at the site of administration as well as selected general signs and symptoms. They should be collected explicitly and systematically during the postvaccination observation period. Subjects should be monitored closely for 20-30 min after vaccination for acute AEs (especially for allergic manifestations). For a parenterally administered vaccine, reactions at the site of injection should be quantitatively measured and graded for severity. For vaccines delivered by other routes (oral, mucosal, transcutaneous), site-specific criteria may be required. Typical periods for collecting solicited AEs are 4-7 days following each dose, though these periods may be modified as additional reactogenicity data are generated for a given vaccine [4]. The occurrence of other AEs during a defined period after vaccination should also be recorded. These should be collected in a nonleading (unsolicited) fashion, for example, asking whether the general health status of the volunteer has changed in any way since the previous visit. Where feasible, the use of diary cards or other data collection tools can be helpful to obtain daily information about AEs during the period after vaccination and the volunteer's next follow-up visit. It is very important to train the volunteers in the standardized use of these tools, especially if quantitative data such as intensity of AE and measurement of injection site reactions are requested. For each AE, start and stop dates must be recorded, and the investigator must determine whether a causal relationship exists between the administration of the vaccine and the onset of the AE.
III.
PRACTICAL CONSIDERATIONS FOR PHASE 1 STUDIES The phase 1 program is characterized by one or more studies that have a relatively small sample size and that ask a limited number of focused questions. The phase 1 program should be designed to select a vaccine dose that is safe and sufficiently well-tolerated to justify exposure to larger study populations in phase 2. The study design should incorporate clear stopping rules for individual subjects and for the study population as a whole that are based on specific objective measurements and that permit either the principal investigator or the medical monitor to discontinue dosing pending review of the data by an independent safety monitoring committee or regulatory authorities [5]. Phase 1 studies are principally designed to determine whether a vaccine has an acceptable margin of safety and induces sufficiently robust and appropriate responses to justify the considerable time and expense required to conduct further clinical trials. It is, therefore, important to extract the most information from the study as efficiently as possible. Timely recruitment of studies is an important part of this goal and is usually a challenge. Investigators routinely overestimate the availability of volunteers to participate in phase 1 and phase 2 trials and, therefore, underestimate the time required to fully enroll the studies. Enrollment rates can be enhanced by careful protocol design that pays close attention to details of eligibility criteria, study complexity, and time demands on study subjects. Sites may need to screen many volunteers
3.TRIAL DESIGN FORVACCINES--A.Clinical Development of New Vaccines
~
for each enrolled subject, and therefore advertisement or study announcement strategies designed to target a large potential audience must be developed. The use of multiple study sites is likely to reduce the enrollment period and can be cost-effective. Dose selection for phase 1 must be determined on the basis of preclinical studies or experience with similar vaccines [6]. Vaccine dose may be limited by antigen availability, formulation requirements, or the sensitivity of assays used to characterize product stability. Although it is beyond the scope of this part of Chapter 3 to comment on every class of vaccines, certain generalizations can be made. The majority of licensed subunit vaccines contain 50 ~tg of protein antigen or less per adult dose, but optimal dose selection must be determined experimentally. Dose-response studies should include a threshold dose below which no or minimal responses are seen, a range over which increasing doses induce more consistent seroconversion or higher mean antibody titers, and a dose above which immune responses plateau. Vaccine antigens that are particulate in nature are generally more immunogenic than soluble proteins. Carbohydrate antigens may have unpredictable immunogenicity and often require conjugation to a protein for optimal immune responses. Adjuvants have been demonstrated to enhance the immune responses to virtually all classes of vaccines, but regulatory agencies are likely to insist that the requirement for their inclusion be proven in the clinic. In these studies, the vaccines formulated with and without adjuvant are tested side by side in a controlled, double-blind fashion. Studies of live-attenuated and DNA plasmid vaccines may require dose ranges extending over several orders of magnitude. Phase 1 vaccine trials should routinely be conducted as blinded studies that include a control group as one of the study arms [7]. This design permits the most unbiased assessment of vaccine safety. Phase I studies frequently will evaluate doses extending over a considerable range, and it is desirable to demonstrate a dose response if possible. This typically requires the inclusion of a dose predicted to be less than optimally immunogenic, a dose representing the likely upper range considered practical, and one or more intermediate doses. Formulation of the vaccine antigen at a relatively high concentration permits dilution of the vaccine "at the bedside" or by the study pharmacist and allows a range of vaccine doses to be studied relatively simply. The initial choice of doses should be based upon preclinical immunogenicity studies, but it should recognize that doses administered to small experimental animals may not scale in a linear fashion and may be misleading when predicting response in humans. More importantly, immunogenicity in mice and rabbits can be notoriously poor as a predictor of immune response in humans, as they may greatly overestimate immunogenicity. Studies in nonhuman primates may be more informative, but the availability of suitable primates is often restricted [8]. Blood collection after vaccination is performed to assess vaccine safety and to measure immune responses. Blood collection for laboratory safety studies should occur just before and within 1 week after vaccination and should target a limited subset of laboratory assays selected for their clinical relevance. Serum or whole blood collection for analysis of immune responses should be collected at baseline, before booster doses are administered, and generally within 7-14 days following a dose. The amount of blood required for
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immunogenicity studies is usually underestimated, especially when assay development is still in progress. Because of their complexity, studies of cellular immune responses are frequently restricted to a subset of subjects or a limited number of sampling times, and explicit instructions and detailed technical training may be required for successful recovery and handling of viable peripheral blood cells. Sampling from other sites, such as collection of mucosal secretions, also requires the development of detailed study procedures. Great care must be taken to properly label, process, store, and ship laboratory samples. The duration of follow-up for phase 1 vaccine studies should be at least 6 months after the final injection, but long-term follow-up to assess the duration of immune responses over several years is often valuable. Investigators should consider building in this option from the beginning to maximize volunteer participation.
IV.
PROCEEDING TO PHASE 2 Once it is clear that the vaccine has acceptable reactogenicity and appears safe and immunogenic in healthy adults, it is appropriate to expand exposure to the vaccine to larger and more diverse populations in the phase 2 program. The evaluation of safety continues throughout the phase 2 program and employs methods that extend and refine the data collected in phase 1. Phase 2 generally includes additional dose-response studies, studies of different formulations, schedule optimization studies, and lot consistency studies. The introduction of a new vaccine into a pediatric population may require a parallel series of clinical trials, including age deescalation studies in which dose and schedules are optimized and studies in which the impact of concomitant immunization with existing pediatric vaccines is evaluated. Phase 2 also may include pilot efficacy trials that can be extremely valuable when planning for phase 3. Schedule optimization involves establishing the optimal timing and number of doses required to induce protection. This process can be greatly facilitated if a correlate of protective immunity is known. Some live attenuated vaccines can induce fully protective responses after a single dose, but others in this class and most subunit vaccines require two or more doses to achieve optimal responses. Experience and experimental evidence suggest that booster injections may be more effective when delivered after immune responses have fallen from peak levels, but identification of the optimal timing for booster injections is often empiric [9]. Phase 2 trials typically enroll several hundred subjects per study, and they are commonly conducted at multiple centers. The scope of the phase 2 program will be driven by the complexity of the clinical questions to be resolved. If the vaccine target is a pediatric or otherwise vulnerable population, additional phase 1-2 safety and immunogenicity studies must be conducted in these groups. In the setting of childhood immunization, it is particularly important to assess the impact of the investigational vaccine on the immune responses to concomitant administration of licensed vaccines and vice versa [10]. One issue that arises during the design of dose escalation and formulation studies involving vaccines that include an adjuvant is whether to hold the
3.TRIAL DESIGN FORVACCINES---A.Clinical Development of NewVaccines
~
amount of adjuvant constant and vary the antigen concentration or to hold the ratio of antigen and adjuvant constant and adjust the volume of vaccine administered. Both approaches have their merits and limitations, and the issue should be anticipated and explored during preclinical studies. Practical concerns such as blinding, minimum and maximum constraints on the amount of vaccine injected (generally 0.25-1.0 ml in adults), and the pluses and minuses of multiple vialings on the one hand and the potential for formulation "at the bedside" on the other should be considered well in advance. Because the emphasis on understanding the safety of the vaccine continues into phase 2, the inclusion of control groups is strongly recommended. The selection of a suitable control may be complex. Whenever a control group is used, the study should be designed as a randomized, double-blind trial. Lot consistency studies are frequently included during the phase 2 program to insure that the safety and immunogenicity of the vaccine is equivalent from lot to lot prior to the start of costly pivotal phase 3 studies. As the demonstration of lot manufacturing and formulation consistency is required for licensure, it therefore may be prudent to conduct phase 2 studies with more than one lot of vaccine. Lot consistency studies may be designed as stand alone "shoot and bleed" safety and immunogenicity studies, or they may be bundled with dose-ranging or pilot efficacy studies. In either case, careful documentation of which lots are used in clinical studies is essential for proper analysis of lot consistency data. Pilot efficacy studies should be strongly considered during phase 2 if the incidence of the disease is sufficiently high to obtain preliminary evidence of vaccine efficacy. In addition to reducing the risks associated with phase 3, they are invaluable for working out critical details such as refining end points and case definitions, improving study procedures and systems, evaluating study sites and grooming investigators, and permitting better sample size estimates for phase 3. For certain infectious diseases, it may be possible to estimate the efficacy of a new vaccine by conducting experimental challenge studies [11]. In these studies, volunteers are recruited with the explicit understanding that an experimental challenge will be conducted once immunization has occurred. Such studies are extremely complex and pose extensive ethical and logistic concerns. They are limited to infectious diseases that are either fully treatable or selflimiting [for example, influenza, respiratory syncytial virus (RSV), rhinovirus, malaria, cholera, shigella, salmonella, gonorrhea, and possibly others]. Challenge doses must be well-characterized, reproducible, and highly standardized, and in certain settings they must be carried out under isolation or containment. Challenge studies must not include vulnerable populations such as children, prisoners, or anyone unable to provide fully informed consent [12]. Because of the greater risks inherent with challenge studies, great care must be taken to avoid inappropriate inducement to participate on the basis of volunteer payment. Estimation of sample sizes for phase 2 pilot efficacy trials to be conducted in a field setting can be quite challenging. Often, investigators will be faced with less than complete data on disease incidence, which may have been collected in a nonintervention setting. Experienced researchers understand that
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the observed disease incidence during a study is almost always lower than expected. Decreased rates of disease may be due to factors such as the implementation of highly specific case definitions, the impact of improved case detection and treatment, and, of course, just bad luck. Although the statistician should always carry out formal sample size calculations, these should be considered estimates at best. A useful rule of thumb is to estimate the expected disease rates in the control arm using the best available data and then reduce this expected rate by up to 50%. Estimate the sample size based on the revised incidence rates and consider doubling the resultant sample size if resources permit. A phase 2 pilot efficacy study designed using this process is likely to be adequately powered and will permit reliable decisions to be made on the design and execution of pivotal phase 3 studies. V.
CONCLUSIONS Phase 1 and phase 2 programs are designed to lay the foundation for proceeding to pivotal phase 3 trials. In the context of a vigorous laboratory research program, they provide for an understanding of the safety and immunogenicity of new vaccines, allow for the refinement of immunologic assays and the identification of clinical correlates of immunity, and provide an opportunity to establish proof of principle on a pilot basis. As such, they must be conducted rigorously and in the context of a dialogue with local and national regulatory agencies. The programs instruct the investigational team on the safety profile and immunogenicity of the vaccine and its optimal dose, formulation, route, and schedule. When feasible, properly executed pilot efficacy studies should help predict the likely outcome of large-scale pivotal trials of vaccine efficacy.
REFERENCES 1. Ellis, R. W. (2001). Technologies for the design, discovery, formulation and administration of vaccines. Vaccine 19(17-19): 2681-2687. Review. 2. ICH Guidelines for Good Clinical Practice (E6), Section 4.5, Compliance with Protocol. Federal Register, May 9, 1997. 3. ICH Guidelines for Good Clinical Practice (E2A), Section 4.5, Clinical Safety Data Management: Definitions and Standards for Expedited Reporting. Federal Register, March 1, 1995. 4. Kester, K. E., McKinney, D. A., Tornieporth, N., Ockenhouse, C. E, Heppner, D. G., Hall, T., Krzych, U., Delchambre, M., Voss, G., Dowler, M. G., Palensky, J., Wittes, J. T., Cohen, J., Ballou, W. R. (2001). Efficacy of recombinant circumsporozoite protein vaccine regimens against experimental Plasmodium falciparum malaria. J. Inf. Dis. 183:640-647. 5. Wittes, J. (1993). Behind closed doors: The data monitoring board in randomized clinical trials. Stat. Med. 12(5-6):419-424). 6. Andre, E E., and Foulkes, M. A. (1998). A phased approach to clinical testing: criteria for progressing from Phase I to Phase II to Phase III studies. Dev. Biol. Stand. 95:57-60. 7. Schulz, K. E, and Grimes, D. A. (2002). Blinding in randomised trials: hiding who got what. Lancet 359(9307):696-700. 8. Gordon, D. M., McGovern, T. W., Krzych, U., Cohen, J. C., Schneider, I., LaChance, R., Heppner, D. G., Hollingdale, M., Slaoui, M., Hauser, P., Voet, P., Sadoff, J. C., and Ballou, W. R. (1995). Safety, immunogenicity and efficacy of a recombinantly produced Plasmodium falciparum circumsporozoite protein / HBsAg subunit vaccine. J. Inf. Diseases 171:1576-1585.
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9. Middleman, A. B., Kozinetz, C. A., Robertson, L. M., DuRant, R. H., and Emans, S. J. (2001). The effect of late doses on the achievement of seroprotection and antibody titer levels with hepatitis b immunization among adolescents. Pediatrics 107 (5): 1065-1069. 10. Schmitt, H. J., Zepp, E, Muschenborn, S., Sumenicht, G., Schuind, A., Beutel, K., Knuf, M., Bock, H. L., Bogaerts, H., and Clemens, R. (1998). Immunogenicity and reactogenicity of a Haemophilus influenzae type b tetanus conjugate vaccine when administered separately or mixed with concomitant diphtheria-tetanus-toxoid and acellular pertussis vaccine for primary and for booster immunizations. Eur. J. Pediatr. 157(3):208-214. 11. Herrington, D. A., Clyde, D. E, Murphy, J. R., Baqar, S., Levine, M. M., do Rosario, V., and Hollingdale, M. R. (1988). A model for Plasmodium falciparum sporozoite challenge and very early therapy of parasitaemia for efficacy studies of sporozoite vaccines. Trop. Geogr. Med. 40(2):124-127. 12. Hoffman, S. L. (1997). Experimental challenge of volunteers with malaria. Ann. Intern. Med. 127(3):233-235.
TRIAL DESIGN FOR VACCINES
PART A. Clinical Development of New Vaccines: Phase 1 and 2 Trials w . RIPLEY B A L L O U Immunology and Infectious Diseases, Clinical Development, Medimmune, Inc., Gaithersburg, Maryland 20878
I.
INTRODUCTION
The scientific basis for the development of new vaccines has accelerated greatly over the past 20 years. Major advances in the understanding of the pathophysiology of infectious diseases and a wealth of revolutionary technologies are expected to greatly enhance the feasibility of immunization against diseases for which vaccines do not currently exist [1]. Although relatively few experimental vaccines will make the transition from preclinical studies into trials involving human subjects, careful planning for clinical development activities can ensure that the most promising candidates are evaluated as efficiently as possible. The clinical development of a new vaccine begins with a plan. The clinical development plan serves as a road map and time line and should lay out, in synopsis form, a series of clinical studies designed to provide the data needed for licensure. The plan should encompass phase 1 through phase 3 activities and identify issues that willneed to be addressed in phase 4. The plan should begin with an explicit description of the proposed indications for which licensure will be pursued. The studies required to obtain the data supporting these indications should be described. Critical go-no-go milestones and alternative strategies should be identified. The majority of the clinical studies described in the development plan can be expected to occur in the phase 1 and phase 2 programs. The phase 1 and phase 2 clinical programs should be expected to overlap with preclinical development. This will require an active dialogue between clinicians and research scientists. The preclinical phase provides the data that The Vaccine Book
Copyright 2003, Elsevier Science (USA). All rights reserved.
85
~6
BALLOU
support the rationale for antigen selection, formulation, delivery route, and dose range to be employed in the clinic. Other preclinical objectives include the identification of an appropriate formulation capable of delivering the predicted dose range, an estimation of the potential need for booster doses, and an adequate preclinical safety database (usually including the site of injection toxicology studies that equal or exceed the doses and number of injections intended for human subjects). Additionally, well-characterized, high-throughput laboratory methods for assessing immunogenicity must be developed to support the clinical program. Clear, succinct, and internally consistent protocols are essential for the success of a clinical development program. The clinical protocol is the definitive reference for study conduct [2]. Protocols should have a limited number of objectives (generally less than three), and these should be ranked as to their priority. The investigators must force themselves to stipulate the questions being asked, the data necessary to answer those questions, and methods that must be employed to collect those data. Integral to this process is the careful design of case report forms (CRFs), which will contain the entire set of data available for analysis at study completion. If the data are not in the CRF, they cannot be analyzed. Similarly, if the data are not going to be analyzed, they should not be collected in the CRE Common mistakes among inexperienced investigators include poorly articulated objectives, too many objectives, and collection of data not required to achieve the objective. The tendency to collect data peripheral to the two or three most important objectives must be resisted. The practice of writing protocols collaboratively should be avoided as it tends to result in redundancies and inconsistencies that may lead to confusion among investigators. Careful selection of investigators having previous experience with vaccine studies and access to suitable study populations and recruitment of a sufficient number of sites to insure rapid study enrollment are important keys to success. Investigator meetings are extremely valuable for insuring that each participating site is fully trained and understands the protocol-required procedures. The mechanics of data collection must be understood in great detail. Distribution and management of the investigational agent at the study sites require careful planning. Management of laboratory specimens for determination of vaccine safety and for measurement of immune responses must be coordinated, and systems must be established for reporting and analysis of study data. The study investigators must be thoroughly familiar with the principles of good clinical practice (GCP) as described in the International Conference on Harmonization (ICH) Guidelines for Good Clinical Practice (Section E6) and Clinical Safety Data Management (Section E2A) [3]. These documents detail the responsibilities that the investigator, sponsor, and investigational review board assume when conducting studies involving human subjects, including the commitment of study investigators to conduct the study strictly according to the protocol. The ICH documents also provide essential information about what must be included in the clinical trial protocol, informed consent document, and investigator's brochure. Vaccine development has a long tradition of investigator-driven research. Beginning with William Jenner and continuing until relatively recently, vaccinologists developed their own vaccines and tested them on themselves and
3.TRIAL DESIGN FORVACCINESmA. Clinical Development of NewVaccines
87
their colleagues. Although current vaccine development still relies heavily on individuals who champion a vaccine strategy, there is now a far greater emphasis on understanding vaccine safety and therefore much greater regulatory oversight of all aspects of vaccine development. Investigators who wish to develop vaccines for the future must be committed to a strong multidisciplinary team approach that includes research and development, clinical development, investigational drug safety, regulatory affairs, biostatistics, and data management and that maintains a dialogue with governmental regulatory agencies. They must also insure that adequate organization and financial and personnel resources are available to drive the program through to completion.
II.
PHASE I:A PRIMARY FOCUS ON SAFETY
The phase i program is the period of clinical development during which a vaccine is introduced for the first time, usually into a study population consisting of healthy adult volunteers who do not have preexisting acquired immunity to the disease that the vaccine is designed to prevent. The choice of a naive population permits a detailed safety evaluation in the absence of preexisting immunity to the vaccine antigen and maximizes the interpretability of elicited immune responses. The evaluation of safety requires the establishment of a baseline prior to receiving the first dose of vaccine and the measurement of changes through serial clinical and laboratory evaluation after immunization. The early clinical development of a new vaccine differs somewhat from that of small molecules and other biologicals where pharmacokinetic and pharmacodynamic studies are essential components of the development program. Because vaccine antigens are administered in discrete, widely spaced, and relatively small amounts, the distribution, accumulation, and elimination of the vaccine antigen are rarely described. In contrast, the local and general toxicity of the formulation and the immune responses generated by the vaccine are studied in great detail. Phase 1 studies generally enroll fewer than 50 subjects and, thus, only permit a first-cut evaluation of vaccine safety using a systematic process of collecting adverse events (AEs). The number of volunteers exposed to the new vaccine will be expanded during the phase 2 program, in which several hundred volunteers can be enrolled per study. The safety data collection process is cumulative; summaries should be updated regularly and described in the investigator's brochure. ICH guideline E2A provides clear definitions for AEs and serious adverse events (SAEs). The emphasis should be on detecting important dose-limiting toxicities, recognizing that only the most frequent AEs and SAEs are likely to be detected in phase 1. An important safety readout in phase 1 vaccine trials is reactogenicity, a measure of the local and general inflammatory responses to one or more components of the formulation. Although there is frequently no direct link between vaccine reactogenicity and immunogenicity, inflammatory processes at the cellular level clearly play a crucial role in the induction of immune responses. Reactogenicity may vary substantially among individuals who
88
BALLOU
develop equivalent immune responses to a vaccine. Reactogenicity is characterized by collecting solicited adverse events. Solicited AEs include local reactions at the site of administration as well as selected general signs and symptoms. They should be collected explicitly and systematically during the postvaccination observation period. Subjects should be monitored closely for 20-30 min after vaccination for acute AEs (especially for allergic manifestations). For a parenterally administered vaccine, reactions at the site of injection should be quantitatively measured and graded for severity. For vaccines delivered by other routes (oral, mucosal, transcutaneous), site-specific criteria may be required. Typical periods for collecting solicited AEs are 4-7 days following each dose, though these periods may be modified as additional reactogenicity data are generated for a given vaccine [4]. The occurrence of other AEs during a defined period after vaccination should also be recorded. These should be collected in a nonleading (unsolicited) fashion, for example, asking whether the general health status of the volunteer has changed in any way since the previous visit. Where feasible, the use of diary cards or other data collection tools can be helpful to obtain daily information about AEs during the period after vaccination and the volunteer's next follow-up visit. It is very important to train the volunteers in the standardized use of these tools, especially if quantitative data such as intensity of AE and measurement of injection site reactions are requested. For each AE, start and stop dates must be recorded, and the investigator must determine whether a causal relationship exists between the administration of the vaccine and the onset of the AE.
III.
PRACTICAL CONSIDERATIONS FOR PHASE 1 STUDIES The phase 1 program is characterized by one or more studies that have a relatively small sample size and that ask a limited number of focused questions. The phase 1 program should be designed to select a vaccine dose that is safe and sufficiently well-tolerated to justify exposure to larger study populations in phase 2. The study design should incorporate clear stopping rules for individual subjects and for the study population as a whole that are based on specific objective measurements and that permit either the principal investigator or the medical monitor to discontinue dosing pending review of the data by an independent safety monitoring committee or regulatory authorities [5]. Phase 1 studies are principally designed to determine whether a vaccine has an acceptable margin of safety and induces sufficiently robust and appropriate responses to justify the considerable time and expense required to conduct further clinical trials. It is, therefore, important to extract the most information from the study as efficiently as possible. Timely recruitment of studies is an important part of this goal and is usually a challenge. Investigators routinely overestimate the availability of volunteers to participate in phase 1 and phase 2 trials and, therefore, underestimate the time required to fully enroll the studies. Enrollment rates can be enhanced by careful protocol design that pays close attention to details of eligibility criteria, study complexity, and time demands on study subjects. Sites may need to screen many volunteers
3.TRIAL DESIGN FORVACCINES--A.Clinical Development of New Vaccines
~
for each enrolled subject, and therefore advertisement or study announcement strategies designed to target a large potential audience must be developed. The use of multiple study sites is likely to reduce the enrollment period and can be cost-effective. Dose selection for phase 1 must be determined on the basis of preclinical studies or experience with similar vaccines [6]. Vaccine dose may be limited by antigen availability, formulation requirements, or the sensitivity of assays used to characterize product stability. Although it is beyond the scope of this part of Chapter 3 to comment on every class of vaccines, certain generalizations can be made. The majority of licensed subunit vaccines contain 50 ~tg of protein antigen or less per adult dose, but optimal dose selection must be determined experimentally. Dose-response studies should include a threshold dose below which no or minimal responses are seen, a range over which increasing doses induce more consistent seroconversion or higher mean antibody titers, and a dose above which immune responses plateau. Vaccine antigens that are particulate in nature are generally more immunogenic than soluble proteins. Carbohydrate antigens may have unpredictable immunogenicity and often require conjugation to a protein for optimal immune responses. Adjuvants have been demonstrated to enhance the immune responses to virtually all classes of vaccines, but regulatory agencies are likely to insist that the requirement for their inclusion be proven in the clinic. In these studies, the vaccines formulated with and without adjuvant are tested side by side in a controlled, double-blind fashion. Studies of live-attenuated and DNA plasmid vaccines may require dose ranges extending over several orders of magnitude. Phase 1 vaccine trials should routinely be conducted as blinded studies that include a control group as one of the study arms [7]. This design permits the most unbiased assessment of vaccine safety. Phase I studies frequently will evaluate doses extending over a considerable range, and it is desirable to demonstrate a dose response if possible. This typically requires the inclusion of a dose predicted to be less than optimally immunogenic, a dose representing the likely upper range considered practical, and one or more intermediate doses. Formulation of the vaccine antigen at a relatively high concentration permits dilution of the vaccine "at the bedside" or by the study pharmacist and allows a range of vaccine doses to be studied relatively simply. The initial choice of doses should be based upon preclinical immunogenicity studies, but it should recognize that doses administered to small experimental animals may not scale in a linear fashion and may be misleading when predicting response in humans. More importantly, immunogenicity in mice and rabbits can be notoriously poor as a predictor of immune response in humans, as they may greatly overestimate immunogenicity. Studies in nonhuman primates may be more informative, but the availability of suitable primates is often restricted [8]. Blood collection after vaccination is performed to assess vaccine safety and to measure immune responses. Blood collection for laboratory safety studies should occur just before and within 1 week after vaccination and should target a limited subset of laboratory assays selected for their clinical relevance. Serum or whole blood collection for analysis of immune responses should be collected at baseline, before booster doses are administered, and generally within 7-14 days following a dose. The amount of blood required for
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immunogenicity studies is usually underestimated, especially when assay development is still in progress. Because of their complexity, studies of cellular immune responses are frequently restricted to a subset of subjects or a limited number of sampling times, and explicit instructions and detailed technical training may be required for successful recovery and handling of viable peripheral blood cells. Sampling from other sites, such as collection of mucosal secretions, also requires the development of detailed study procedures. Great care must be taken to properly label, process, store, and ship laboratory samples. The duration of follow-up for phase 1 vaccine studies should be at least 6 months after the final injection, but long-term follow-up to assess the duration of immune responses over several years is often valuable. Investigators should consider building in this option from the beginning to maximize volunteer participation.
IV.
PROCEEDING TO PHASE 2 Once it is clear that the vaccine has acceptable reactogenicity and appears safe and immunogenic in healthy adults, it is appropriate to expand exposure to the vaccine to larger and more diverse populations in the phase 2 program. The evaluation of safety continues throughout the phase 2 program and employs methods that extend and refine the data collected in phase 1. Phase 2 generally includes additional dose-response studies, studies of different formulations, schedule optimization studies, and lot consistency studies. The introduction of a new vaccine into a pediatric population may require a parallel series of clinical trials, including age deescalation studies in which dose and schedules are optimized and studies in which the impact of concomitant immunization with existing pediatric vaccines is evaluated. Phase 2 also may include pilot efficacy trials that can be extremely valuable when planning for phase 3. Schedule optimization involves establishing the optimal timing and number of doses required to induce protection. This process can be greatly facilitated if a correlate of protective immunity is known. Some live attenuated vaccines can induce fully protective responses after a single dose, but others in this class and most subunit vaccines require two or more doses to achieve optimal responses. Experience and experimental evidence suggest that booster injections may be more effective when delivered after immune responses have fallen from peak levels, but identification of the optimal timing for booster injections is often empiric [9]. Phase 2 trials typically enroll several hundred subjects per study, and they are commonly conducted at multiple centers. The scope of the phase 2 program will be driven by the complexity of the clinical questions to be resolved. If the vaccine target is a pediatric or otherwise vulnerable population, additional phase 1-2 safety and immunogenicity studies must be conducted in these groups. In the setting of childhood immunization, it is particularly important to assess the impact of the investigational vaccine on the immune responses to concomitant administration of licensed vaccines and vice versa [10]. One issue that arises during the design of dose escalation and formulation studies involving vaccines that include an adjuvant is whether to hold the
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amount of adjuvant constant and vary the antigen concentration or to hold the ratio of antigen and adjuvant constant and adjust the volume of vaccine administered. Both approaches have their merits and limitations, and the issue should be anticipated and explored during preclinical studies. Practical concerns such as blinding, minimum and maximum constraints on the amount of vaccine injected (generally 0.25-1.0 ml in adults), and the pluses and minuses of multiple vialings on the one hand and the potential for formulation "at the bedside" on the other should be considered well in advance. Because the emphasis on understanding the safety of the vaccine continues into phase 2, the inclusion of control groups is strongly recommended. The selection of a suitable control may be complex. Whenever a control group is used, the study should be designed as a randomized, double-blind trial. Lot consistency studies are frequently included during the phase 2 program to insure that the safety and immunogenicity of the vaccine is equivalent from lot to lot prior to the start of costly pivotal phase 3 studies. As the demonstration of lot manufacturing and formulation consistency is required for licensure, it therefore may be prudent to conduct phase 2 studies with more than one lot of vaccine. Lot consistency studies may be designed as stand alone "shoot and bleed" safety and immunogenicity studies, or they may be bundled with dose-ranging or pilot efficacy studies. In either case, careful documentation of which lots are used in clinical studies is essential for proper analysis of lot consistency data. Pilot efficacy studies should be strongly considered during phase 2 if the incidence of the disease is sufficiently high to obtain preliminary evidence of vaccine efficacy. In addition to reducing the risks associated with phase 3, they are invaluable for working out critical details such as refining end points and case definitions, improving study procedures and systems, evaluating study sites and grooming investigators, and permitting better sample size estimates for phase 3. For certain infectious diseases, it may be possible to estimate the efficacy of a new vaccine by conducting experimental challenge studies [11]. In these studies, volunteers are recruited with the explicit understanding that an experimental challenge will be conducted once immunization has occurred. Such studies are extremely complex and pose extensive ethical and logistic concerns. They are limited to infectious diseases that are either fully treatable or selflimiting [for example, influenza, respiratory syncytial virus (RSV), rhinovirus, malaria, cholera, shigella, salmonella, gonorrhea, and possibly others]. Challenge doses must be well-characterized, reproducible, and highly standardized, and in certain settings they must be carried out under isolation or containment. Challenge studies must not include vulnerable populations such as children, prisoners, or anyone unable to provide fully informed consent [12]. Because of the greater risks inherent with challenge studies, great care must be taken to avoid inappropriate inducement to participate on the basis of volunteer payment. Estimation of sample sizes for phase 2 pilot efficacy trials to be conducted in a field setting can be quite challenging. Often, investigators will be faced with less than complete data on disease incidence, which may have been collected in a nonintervention setting. Experienced researchers understand that
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the observed disease incidence during a study is almost always lower than expected. Decreased rates of disease may be due to factors such as the implementation of highly specific case definitions, the impact of improved case detection and treatment, and, of course, just bad luck. Although the statistician should always carry out formal sample size calculations, these should be considered estimates at best. A useful rule of thumb is to estimate the expected disease rates in the control arm using the best available data and then reduce this expected rate by up to 50%. Estimate the sample size based on the revised incidence rates and consider doubling the resultant sample size if resources permit. A phase 2 pilot efficacy study designed using this process is likely to be adequately powered and will permit reliable decisions to be made on the design and execution of pivotal phase 3 studies. V.
CONCLUSIONS Phase 1 and phase 2 programs are designed to lay the foundation for proceeding to pivotal phase 3 trials. In the context of a vigorous laboratory research program, they provide for an understanding of the safety and immunogenicity of new vaccines, allow for the refinement of immunologic assays and the identification of clinical correlates of immunity, and provide an opportunity to establish proof of principle on a pilot basis. As such, they must be conducted rigorously and in the context of a dialogue with local and national regulatory agencies. The programs instruct the investigational team on the safety profile and immunogenicity of the vaccine and its optimal dose, formulation, route, and schedule. When feasible, properly executed pilot efficacy studies should help predict the likely outcome of large-scale pivotal trials of vaccine efficacy.
REFERENCES 1. Ellis, R. W. (2001). Technologies for the design, discovery, formulation and administration of vaccines. Vaccine 19(17-19): 2681-2687. Review. 2. ICH Guidelines for Good Clinical Practice (E6), Section 4.5, Compliance with Protocol. Federal Register, May 9, 1997. 3. ICH Guidelines for Good Clinical Practice (E2A), Section 4.5, Clinical Safety Data Management: Definitions and Standards for Expedited Reporting. Federal Register, March 1, 1995. 4. Kester, K. E., McKinney, D. A., Tornieporth, N., Ockenhouse, C. E, Heppner, D. G., Hall, T., Krzych, U., Delchambre, M., Voss, G., Dowler, M. G., Palensky, J., Wittes, J. T., Cohen, J., Ballou, W. R. (2001). Efficacy of recombinant circumsporozoite protein vaccine regimens against experimental Plasmodium falciparum malaria. J. Inf. Dis. 183:640-647. 5. Wittes, J. (1993). Behind closed doors: The data monitoring board in randomized clinical trials. Stat. Med. 12(5-6):419-424). 6. Andre, E E., and Foulkes, M. A. (1998). A phased approach to clinical testing: criteria for progressing from Phase I to Phase II to Phase III studies. Dev. Biol. Stand. 95:57-60. 7. Schulz, K. E, and Grimes, D. A. (2002). Blinding in randomised trials: hiding who got what. Lancet 359(9307):696-700. 8. Gordon, D. M., McGovern, T. W., Krzych, U., Cohen, J. C., Schneider, I., LaChance, R., Heppner, D. G., Hollingdale, M., Slaoui, M., Hauser, P., Voet, P., Sadoff, J. C., and Ballou, W. R. (1995). Safety, immunogenicity and efficacy of a recombinantly produced Plasmodium falciparum circumsporozoite protein / HBsAg subunit vaccine. J. Inf. Diseases 171:1576-1585.
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9. Middleman, A. B., Kozinetz, C. A., Robertson, L. M., DuRant, R. H., and Emans, S. J. (2001). The effect of late doses on the achievement of seroprotection and antibody titer levels with hepatitis b immunization among adolescents. Pediatrics 107 (5): 1065-1069. 10. Schmitt, H. J., Zepp, E, Muschenborn, S., Sumenicht, G., Schuind, A., Beutel, K., Knuf, M., Bock, H. L., Bogaerts, H., and Clemens, R. (1998). Immunogenicity and reactogenicity of a Haemophilus influenzae type b tetanus conjugate vaccine when administered separately or mixed with concomitant diphtheria-tetanus-toxoid and acellular pertussis vaccine for primary and for booster immunizations. Eur. J. Pediatr. 157(3):208-214. 11. Herrington, D. A., Clyde, D. E, Murphy, J. R., Baqar, S., Levine, M. M., do Rosario, V., and Hollingdale, M. R. (1988). A model for Plasmodium falciparum sporozoite challenge and very early therapy of parasitaemia for efficacy studies of sporozoite vaccines. Trop. Geogr. Med. 40(2):124-127. 12. Hoffman, S. L. (1997). Experimental challenge of volunteers with malaria. Ann. Intern. Med. 127(3):233-235.