Tissue and tissue fluid penetration of antibiotics

ANNLDO 6(9) 69--76, 1989

ISSN 0738-1751

VOLUME 6, NUMBER 9, SEPTEMBER 1989

EDITORIAL BOARD Editor

Associate Editors

DANIEL AMSTERDAM, PhD

STEVEN L. BARRIERE, PharmD

CLYDE THORNSBERRY, PhD

State University of New York at Buffalo and Erie County Medical Center Buffalo, New York

UCLA Medical Center Los Angeles, California

Center for Infectious Diseases Centers for Disease Control Atlanta, Georgia

RONALD N. JONES, MD University of Iowa Hospitals and Clinics Iowa City, Iowa

HAROLD C. NEU, MD College of Physicians and Surgeons, Columbia University, New York, New York

LOWELL S. YOUNG, MD Kuzell Institute for Arthritis and Infectious Diseases Medical Research Institute of San Francisco Pacific Presbyterian Medical Center San Francisco, California

TISSUE A N D TISSUE FLUID PENETRATION OF ANTIBIOTICS EDITOR'S NOTE

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D. AMSTERDAM Tissue and Fluid Penetration of Antibiotics 69 M. LEBEL

M A R C LEBEL Ecole De Pharmacie Universite Laval Quebec, Canada

REPORTS FROM TIlE LITERATURE

The Development of Resistance During Antimicrobial Therapy D. AMSTERDAM Interactions Between Microorganisms, Antimicrobics, and Cells E. A. G O - - S K I

EDITOR'S NOTE

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Achieving microbiologically active concentrations of antibiotics at the site of infection remains a major factor in successful antibiotic therapy. This principle has provided the impetus for a legion of studies on tissue penetration of antimicrobial agents in almost every imaginable tissue or tissue fluid. Antibiotics are a unique class of

model for tissue fluid penetration, and sample calculations for the [3lactams and fluoroquinolones are The concepts of tissue and tissue supplied. fluid penetration are presented in this issue of The Antimicrobic NewsTwo different approaches for letter. Dr. Marc LeBel limits his monitoring the frequency of the discussion to those compartments development of antimicrobial resis(bone, muscle, skin, ascites, tance is presented in another secpleural, and peritoneal fluid) where tion. As might be expected, the passive diffusion governs antibiotic microorganism most frequently enpenetration. Sites with active countered in surveying resistance transport mechanisms (e.g., vitdevelopment is Pseudomonas aerureous humor or renal tubules and ginosa. In the latter section, Dr. specialized exclusion systems such E. A. Gorzynski reviews the interas CSF or walled-off abscesses) are action of antimicrobial agents, miexcluded. The author provides a croorganisms, and cells.

ELSEVIER

therapeutic agents in this regard. However, designing optimal antibiotic dosage regimens must include several other factors involved in bacterial eradication at the infection site: pathogen susceptibility, bacterial density and rate of multiplication, host defenses, subpopulation of resistant bacteria, post-antibiotic effect, and others. This paper attempts to summarize the concepts behind antibiotic tissue penetration, concepts that are often overlooked in many papers published on this topic. Discussion will be limited to tissue or tissue fluid where only passive diffusion governs antibiotic penetration such as bone, muscle, skin and ascites, pleural effusion, and peritoneal fluid. Sites with active transport mechanisms such as vitreous humor of the eye or renal tubules, sites with specialized exclusion systems such as CSF, sites with barriers to diffusion such as a walled-off abscess, and excretory organs such as the biliary and urinary tracts have been excluded.

TISSUE PENETRATION It is not well recognized that in tissue the total antibiotic concentration must represent the mean of o 1751,s9 o.oo + 2.20

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the extracellular and intracellular antibiotic levels. Lipophilic antibiotics such as tetracyclines, macrolides, and fluoroquinolones penetrate the cells and consequently achieve tissue concentrations in excess of blood levels. More hydrophilic antibiotics are excluded from intact cells and remain extracellular in the vascular and extravascular spaces. Examples of the latter g r o u p are [3-1actam antibiotics and vancomycin. Although aminoglycosides m a y belong to this group, they also accumulate t h r o u g h active transport into kidney tubular cells over time. The tissue concentrations of [3-1actam antibiotics are consistently f o u n d to be low in relation to the blood levels. 1-3 Such data suggest that these antibiotics have a restricted capacity to pass into tissues. Therefore, higher dosages of 13lactam antibiotics have been advocated for treatment of tissue infections to circumvent this purported poor tissue penetration. 4 In an excised tissue sample (pancreas, for example), the extracellular fluid (ECF) consists of 20% of the mass, and all [3-1actam antibiotic molecules are confined to this 20%. The rest is c o m p o s e d of intracellular fluid (ICF) (60%) and solids (20%), i.e., m e m b r a n e s and cellular organelles. Schentag et al 2 suggested a m e t h o d to calculate drug concentrations in homogenized tissue samples taken in the postdistributive phase (30 minutes after the e n d of an i.v. infusion). The calculation is illustrated in Table 1. Total tissue concentrations of ~-lactam antibiotics are estimated at 8.8 ~g/g with a corresponding 40 ~g/mL in serum.

THE A N T I M I C R O B I C NEWSLETTER, V O L U M E 6, N U M B E R 9, SEPTEMBER 1989

TABLE 1. Calculation Example of [3-Lactam Concentrations in an Excised Tissue Sample Mass Conc. Amount Site (%, V/V) x (~g/mL) = (~g/g) Solids ICF Serum ECF Total tissue concentration

20 60 4 16

x x x x

0.01 0.01 40.0 40.0

= = = = =

0.2 0.6 1.6 6.4 8.8

V = volume

In this example, 20% of the tissue mass contains almost 100% of the [3-1actam antibiotics. The remaining 80%, with very little antibiotic, contributes to lower the final tissue concentration. The v o l u m e of distribution of ~-lactam antibiotics (0.20-0.25 L/kg) equals the ECF, which indicates that all the fluid spaces outside the cells have the same steady-state concentration. Contrary to general belief, it is possible to predict tissue concentration of ~-lactam antibiotics as long as we k n o w the contribution of solids and intracellular fluid to total mass of a specific tissue. This ratio is different for tissues in different b o d y sites. For example, extracellular fluids of healthy bone represent approximately 5% of the bone weight and 10% of the weight for skeletal muscle. 3

W h e n these data are c o m p a r e d to ~-lactam efficacy, low tissue antibiotic levels do not correlate with low clinical cure rate. Because most bacteria are located at infection sites in extracellular fluids, they are exposed to extracellular antibiotic concentrations that are identical to those in serum at steady-state. 1 More lipophilic antibiotics penetrate into cells to reach concentrations as m u c h as two to three times greater than in blood, as in the case of fluoroquinolones. Table 2 illustrates the same calculation for the tissue penetration of fluoroquinolones for a total tissue concentration of 7.2 ~g/g and a corresponding whole blood concentration of 4.0 ~g/mL. In this example, the tissue to blood or serum ratio exceeds 1:1 (ca. 1.8:1) as the antibiotic dis-

TABLE 2. Calculation Example of Fluoroquinolone Concentrations in an Excised Tissue Sample Site Solids ICF Blood ECF Total tissue concentration

Mass (%, V/V) 20 60 4 16

x x x x x

Conc. (lag/mL) 20.0 4.0 4.0 4.0

= = = = = =

Amount (p,g/g) 4.00 2.40 0.16 0.64 7.2

V = volume

The Antimicrobic Newsletter (ISSN 0738-1751) is issued monthly in one indexed volume per year by Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, New York, NY 10010. Printed in USA (at Fame Avenue, Hanover, PA 17331). Subscription price per year: $75.00. Outside of the USA, Canada and Mexico, add $33.00. Second-class postage pending at New York, NY, and at additional mailing offices. Postmaster: Send address changes to The Antimicrobic Newsletter, Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, New York, NY 10010. NOTICE: No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. No suggested test or procedure should be carried out unless, in the reader's judgment, its risk is justified. Because of rapid advances in the medical sciences, we recommend that the independent verification of diagnoses and drug dosages should be made. Discussions, views and recommendations as to medical procedures, choice of drugs and drug dosages are the responsibility of the authors.

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THE ANTIMICROBIC NEWSLETI'ER, VOLUME 6, NUMBER 9, SEPTEMBER 1989

tributes homogeneously in ECF and less homogeneously throughout intracellular organelles. A corollary to high tissue concentration of antibiotics such as fluoroquinolones is a low extraceUular concentration that justifies higher dosages with these antibiotics for the treatment of extracellular infections. The most notable among intracellular pathogens include Mycoplasma, Listeria, and Legionella. Most microorganisms invade tissue via the interstitial fluid.

different physiologic characteristics that influence the rate and the extent of antibiotic penetration. The surface area (SA) where passive diffusion can occur and its relation to the volume (V) of fluids have been identified to be of primary importance to fluid penetration. 6,13 Vascularization and actual perfusion of the different fluid spaces are other factors playing a primary role in antibiotic penetration.

MODELS OF TISSUE FLUID PENETRATION PENETRATION OF EXTRACELLULAR FLUIDS

As noted previously, penetration of the different groups of antibiotics into extracellular fluids results in free (nonprotein-bound) drug concentrations similar to free drug serum concentrations. Fluid spaces such as peritoneal, pleural, and synovial fluids achieve fluid to serum ratios approaching 1:1 at steadystate. 1-3,5,6 It is therefore possible to predict extracellular fluid penetration for most antibiotics based on free drug serum levels. However, some minor differences exist between the different groups of antibiotics, between agents in the same group, and between some fluid spaces. Partition coefficients (lipo- versus hydrophilicity) and pKa distinguish the tissue fluid penetration between different antibiotic groups and different agents within a group. Protein-binding has long been identified as a limit to the rate of penetration, but not to the extent of penetration of antibiotics. 7-11 Since extracellular fluids as well as tissues contain proteins, antibiotic binding occurs in these spaces. A reservoir effect is often described for some antibiotics, more for those exhibiting slower clearance from the body. The effect overcomes the delay in fluid penetration, as in the case of ceftriaxone. 7,12 The different fluids spaces have

In order to better study the tissue and tissue fluid penetration of antibiotics in humans along with their pharmacokinetic behavior, several models of "interstitial fluid" penetration have been designed. Blister fluid models, either induced by cantharidin or by suction, allow nontraumatic access to inflammatory and noninflammatory interstitial fluid, respectivelyJ 4,1s Blister models equilibrate slowly with serum and, because of their small surface area to volume ratio (SA/V) and resultant slow equilibrium, they are not considered representative of interstitial fluid. 2,5 In addition, with suction-induced blisters, healing at the blister base starting soon after blister formation impedes diffusion of antibiotics after a certain period (approximately 24 hours). 12 However, these models are highly reproducible, allow comparison between different antibiotics, 14,~6and probably represent burn wounds and small abscesses. Small filter paper discs applied on denuded dermis and implanted cotton threads are two models of interstitial fluid penetration, which represent more closely the rate of penetration into interstitial fluid because they equilibrate rapidly with serum. 17,:8 Unfortunately, most of these tissue fluid penetration models are not ideal for antibiotics from a pharmacokinetic and

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pharmacodynamic point of view, but they are useful in elucidating the factors that govern this process.

IS ANTIBIOTIC PENETRATION MODIFIED BY INFECTION? The infectious process itself is likely to change the relative proportion of extracellular fluid to total mass of tissue. Local changes in the blood circulation, increased volume of extracellular fluid (tissue edema), fibrin barriers or abscess formation, and drug binding to necrotic tissue constituents are examples of alterations occurring in an infected t i s s u e 1-3,6 compared to the natural state. In most acute infections, it is unlikely that these changes will be large enough to have a substantial influence on tissue pharmacokinetics. This is supported by studies of soft tissue infection, which reveal no difference in pharmacokinetic behavior between serum and soft tissues. 6 More profound changes in tissue architecture may, however, produce lower antibiotic concentrations, such as w i t h walled-off purulent abscesses or grossly edematous soft tissues. Regardless, it is believed that the antimicrobial/microbe "battle" is going on in the periphery of an abscess, because the largest numbers of viable bacteria are found here. There is a definite need for more research in this field.

CONCLUSIONS Studies on penetration of new antibiotics in samples of human tissue or tissue fluid will continue to pervade the antibiotic literature. Although these studies are useful to assess the relationships between drug protein binding and activity in these tissues, we should be focusing on the correlation of efficacy with drug concentration in easily accessible clinical materials such as serum. This will be achieved when a more complete knowledge is 0738-1751/89/$0.00 + 2.20

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gained of the contribution of solids and intra- and extracellular fluids of each tissue site. The influence of infectious process and other conditions on these estimates of different tissue components will also have to be determined.

REFERENCES

1. Cars O, Ogren S: Antibiotic tissue concentrations: Methodological aspects and interpretation of results. Scand J Infect Dis 44:7-15, 1985. 2. Schentag JJ, Swanson DJ, Smith IL: Dual individualization: Antibiotic dosage calculation from the integration of in vitro pharmacodynamics and in vivo pharmacokinetics. J Antimicrob Chemother 15(Suppl. A):47-57, 1985. 3. Barza M, Cuchural G: General principles of antibiotic tissue penetration. J Antimicrob Chemother 15(Suppl. A):59-75, 1985. 4. Kunin CM: Dosage schedules of antimicrobial agents: A historical review. Rev Infect Dis 3:4-11, 1981.

THE ANTIMICROBIC NEWSLETTER, VOLUME 6, NUMBER 9, SEPTEMBER 1989

5. Schentag JJ: Clinical significance of antibiotic tissue penetration. Clin Pharm 16(Suppl. 1):25-31, 1989. 6. Ryan DM, Cars O, Joffstedt B: The use of antibiotic serum levels to predict concentrations in tissues. Scand J Infect Dis 18:381-388, 1986. 7. Craig WA, Vogelman B: Changing concepts and new applications of antibiotic pharmacokinetics. Am J Med 77:24-28, 1984. 8. Wise R, et al: The influence of protein binding upon tissue fluid levels of six ~-lactam antibiotics. J Infect Dis 142:77-82, 1980. 9. Bergan T, Engeset A, Olszewski W: Does serum protein binding inhibit tissue penetration of antibiotics? Rev Infect Dis 9:713-718, 1987. 10. Wise R: The clinical relevance of protein binding and tissue concentrations in antimicrobial therapy. Clin Pharmacokin 11:470-482, 1986. 11. Ogren S, Cars O: Importance of drug-protein interactions and protein concentrations for antibiotic levels in serum and tissue fluid. Scand J Infect Dis (Suppl.)44:34-40, 1985.

REPORTS FROM THE LITERATURE The Development of Resistance During Antimicrobial Therapy D. AMSTERDAM Clinicians all too frequently encounter the refractory nature of an offending microorganism during the course of a patient's antimicrobial therapy. Although resistance of the infecting organism that develops during the course of a therapeutic regimen is a recognized occurrence, it is anecdotal and not well documented. Two reports discussed here attempt to document resistance encountered during the course of therapy. In an extensive review, Milatovic and BravenyI assessed the devel-

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opment of resistance during therapy as determined by analysis of publications in the literature; another report evidenced the development of resistance to the aminoglycoside amikacin during the course of hospitalized patients.2 In their overview, Milatovic and Braveny surveyed the literature from 1945 through 1986. The authors reviewed such wide ranging articles as the "Susceptibility of Wounds to Penicillin" published in War Medicine in 1945, 3 the resistance to chloramphenicol that developed during the treatment of typhoid in 1950,4 and a

12. LeBel M, Gr6goire S, Caron M, Bergeron MG: Difference in blister fluid penetration after single and multiple doses of ceftriaxone. Antimicrob Agents Chemother 28:123127, 1985. 13. Ryan M: Influence of surface area/ volume ratio on the kinetics of antibiotics in different tissues and tissue fluids. Scand J Infect Dis (Suppl)44:24-33, 1985. 14. Wise R: Methods for evaluating the penetration of f~-lactamantibiotics into tissues. Rev Infect Dis 8(Suppl 3):$325-$332, 1986. 15. Mazzei T, Periti P: Tissue distribution of antimicrobial drugs. J Chemother 1:75-79, 1989. 16. LeBelM: Pharmacokinetics of oral fluoroquinolones. ISI Atlas of Science: Pharmacology 2:196-210, 1988. 17. Shyu WC, Quintiliani R, Nightingale CH: An improved method to determine interstitial fluid pharmacokinetics. J Infect Dis 152:13281331, 1985. 18. Ryan DM, Cars O: A problem in the interpretation of ~-lactam antibiotic levels in tissues. J Antimicrob Chemother 12:281-284, 1983.

1946 article by Finland regarding the development of resistance to streptomycin treatment. 5 The investigators acknowledged the difficulty in evaluating the literature and developed certain guidelines for determining the frequency of resistance development from the published works. For example, in the case of the older antibiotics, the frequency of resistance development was not subject to scrutiny as the articles were simply case reports. The ground rules for documented cases of resistance had to follow the following criteria: • Only prospective clinical studies were evaluated and had to include a relatively large number of patients. • The resistant isolate, i.e., the isolate that developed resistance subsequent to the initial suscep-

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