Pre- and postexposure chemoprophylaxis: evidence that 3'-azido-3'-dideoxythymidine inhibits feline leukemia virus disease by a drug-induced vaccine response

Efficacy of antiviral chemotherapy for retrovirus-infected cats: What does the current literature tell us?

GLOBAL IMPORTANCE

The two feline retroviruses, feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV), are global and widespread, but differ in their potential to cause disease.

VIRAL INFECTION - FIV

FIV, a lentivirus that shares many properties with human immunodeficiency virus (HIV), can cause an acquired immune deficiency syndrome, which predisposes cats to other infections, stomatitis, neurological disorders and tumours. Although secondary infections are common, specific opportunistic infections or acquired immunodeficiency virus-defining infections, such as those that occur with HIV, are not commonly reported in FIV-infected cats. In most naturally infected cats, FIV does not cause a severe clinical syndrome; with appropriate care, FIV-infected cats can live many years before succumbing to conditions unrelated to their FIV infection. Thus, overall survival time is not necessarily shorter than in uninfected cats, and quality of life is usually high over many years or lifelong.

VIRAL INFECTION - FELV

FeLV, an oncornavirus, is more pathogenic than FIV. Historically, it was considered to account for more disease-related deaths and clinical syndromes in cats than any other infectious agent. Recently, the prevalence and importance of FeLV have been decreasing, mainly because of testing and eradication programmes and the use of FeLV vaccines. Progressive FeLV infection can cause tumours, bone marrow suppression and immunosuppression, as well as neurological and other disorders, and leads to a decrease in life expectancy. However, with appropriate care, many FeLV-infected cats can also live several years with a good quality of life.

PRACTICAL RELEVANCE

A decision regarding treatment or euthanasia should never be based solely on the presence or absence of a retrovirus infection. Antiviral chemotherapy is of increasing interest in veterinary medicine, but is still not used commonly.

EVIDENCE BASE

This article reviews the current literature on antiviral chemotherapy in retrovirus-infected cats, focusing on drugs that are currently available on the market and, thus, could potentially be used in cats.

A trial with 3′-azido-2′,3′-dideoxythymidine and human interferon-α in cats naturally infected with feline leukaemia virus

Feline leukaemia virus (FeLV) infection is still one of the leading causes of infection-related deaths in domestic cats. Treatment with various drugs has been attempted, but none has resulted in cure or complete virus elimination. Human interferon-α2a (huIFN-α2a) and 3′-azido-2′,3′-dideoxythymidine (AZT) have been proven to decrease antigenaemia in cats infected experimentally with FeLV. The purpose of this study was to assess the efficacy of huIFN-α2a, AZT and a combination of both drugs in cats infected naturally with FeLV in a placebo-controlled double-blinded trial. Fourty-four FeLV-infected cats in which free FeLV p27 antigen was detected in serum by enzyme-linked immunosorbent assay were included in the study. Cats were assigned to one of four treatment groups that received either high dose huIFN-α2a (105 IU/kg q24h; 12 cats), AZT (5 mg/kg q12h; 10 cats, both of these treatments (12 cats) or placebo (10 cats). All cats were treated for 6 weeks. Clinical variables, including stomatitis, and laboratory parameters, such as CD4+ and CD8+ counts and serum FeLV p 27 antigen concentration, were recorded throughout the treatment period. No significant difference among the groups was observed during the treatment period for any of the parameters. Aside from anaemia in one cat treated with AZT, no adverse effects were observed. It was not possible to demonstrate efficacy of huIFN-α2a or AZT alone or together in cats infected naturally with FeLV when given according to this regimen for 6 weeks; however, no notable side effects were detected.

Comparison of DNA polymerase activities between recombinant feline immunodeficiency and leukemia virus reverse transcriptases

In this study, we present enzymatic differences found between recombinant RTs derived from feline leukemia virus and feline immunodeficiency virus. Firstly, FIV RT showed low steady state K(m) values for dNTPs compared to FeLV RT. Consistent with this, FIV RT synthesized DNA more efficiently than FeLV RT at low dNTP concentrations. We observed similar concentration-dependent activity differences between other lentiviral (HIV-1 and SIV) and non-lentiviral (MuLV and AMV) RTs. Second, FeLV RT showed less efficient misincorporation with biased dNTP pools and mismatch primer extension capabilities, compared to FIV RT. In M13mp2 lacZalpha forward mutation assays, FeLV RT displayed approximately 11-fold higher fidelity than FIV RT. Finally, FeLV RT was less sensitive to 3TCTP and ddATP than FIV RT. This study represents the comprehensive enzymatic characterization of RTs from a lentivirus and a non-lentivirus retrovirus from the same host species. The data presented here support enzymatic divergences seen among retroviral RTs.

Management of occupational and nonoccupational postexposure HIV prophylaxis

The principles of managing patients with recent HIV exposures are similar whether the exposure occurs in an occupational or nonoccupational setting. For both settings, clinicians should 1) assess the likelihood that HIV and other bloodborne viruses will be transmitted as a consequence of the exposure; 2) advise the patient about the risks and benefits of treatment; 3) choose an appropriate antiretroviral treatment regimen (if the decision is made to treat); 4) screen for other illnesses that may complicate treatment or follow-up; 5) counsel patients about the importance of adhering to treatment; 6) promote safe-sex practices and methods to avoid future exposures; 7) follow the patient for potential side effects of treatment; 8) provide follow-up care including repeat HIV testing for seroconversion, surveillance for primary HIV infection, and reinforcement of counseling messages.

Pharmacokinetics of zidovudine in cats

OBJECTIVE

To characterize the pharmacokinetics of zidovudine (AZT) in cats.

ANIMALS

6 sexually intact 9-month-old barrier-reared domestic shorthair cats.

PROCEDURE

Cats were randomly alloted into 3 groups, and zidovudine (25 mg/kg) was administered i.v., intragastrically (i.g.), and p.o. in a 3-way crossover study design with 2-week washout periods between experiments. Plasma samples were collected for 12 hours after drug administration, and zidovudine concentrations were determined by high-performance liquid chromatography. Maximum plasma concentrations (Cmax), time to reach Cmax (Tmax), and bioavailability were compared between i.g. and p.o. routes. Area under the curve (AUC) and terminal phase half-life (t(1/2)) among the 3 administration routes were also compared.

RESULTS

Plasma concentrations of zidovudine declined rapidly with t(1/2) of 1.4 +/- 0.19 hours, 1.4 +/- 0.16 hours, and 1.5 +/- 0.28 hours after i.v., i.g., and p.o. administration, respectively. Total body clearance and steady-state volume of distribution were 0.41 +/- 0.10 L/h/kg and 0.82 +/- 0.15 L/kg, respectively. Mean Tmax for i.g. administration (0.22 hours) was significantly shorter than Tmax for p.o. administration (0.67 hours). The AUC after i.v. and p.o. administration was 64.7 +/- 16.6 mg x h/L and 60.5 +/- 17.0 mg x h/L, respectively, whereas AUC for the i.g. route was significantly less at 42.5 +/- 9.41 mg x h/L. Zidovudine was well absorbed after i.g. and p.o. administration with bioavailability values of 70 +/- 24% and 95 +/- 23%, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Cats had slower clearance of zidovudine, compared with other species. Plasma concentrations of zidovudine were maintained above the minimum effective concentration for inhibiting FIV replication by 50% (0.07 microM [0.019 microg/mL] for wild-type FIV clinical isolate) for at least 12 hours after i.v., i.g., or p.o. administration.

Antiretroviral efficacy of a 98% solution of glycerol or ethylene oxide for inactivation of feline leukemia virus in bone

OBJECTIVE

To determine whether infectious retrovirus was inactivated in bones from FeLV-infected cats after ethylene oxide (ETO) sterilization or preservation in a 98% solution of glycerol in an in vitro cell culture system.

SAMPLE POPULATION

Metatarsal bones obtained from 5 FeLV-infected cats and cultured with feline fibroblast cells.

PROCEDURE

Metatarsal bones were treated with 100% ETO, a 98% solution of glycerol, or left untreated. Twenty-five flasks of feline fibroblast cells were assigned to 5 groups: negative control, positive control, ETO-treated bone, glycerol-treated bone, and untreated bone with 5 replicates/group for 4 passages. Media and cell samples were harvested from every flask at each passage to measure FeLV p27 antigen and the number of copies of provirus per 100 ng of DNA, respectively.

RESULTS

All negative control and ETO-treated group replicates were negative for FeLV p27 antigen and provirus throughout the study. All positive control group replicates were positive for FeLV p27 antigen and provirus at passages 1 to 4. Untreated bone group replicates were positive for FeLV p27 antigen at passages 3 and 4 and provirus beginning at passage 2. Glycerol-treated group replicates had delayed cell replication and were negative for FeLV p27 antigen and provirus at passages 1 to 4 and 2 to 4, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Ethylene oxide sterilization of bone from FeLV-infected cats appeared to abrogate transmission of infectious retrovirus and effectively sterilized bone allografts. Impact for Human Medicine-Additional studies to confirm effectiveness of ETO treatment of allograft tissues for prevention of pathogen transmission via transplantation are warranted.

Postexposure prophylaxis in children and adolescents for nonoccupational exposure to human immunodeficiency virus

Exposure to human immunodeficiency virus (HIV) can occur in a number of situations unique to, or more common among, children and adolescents. Guidelines for postexposure prophylaxis (PEP) for occupational and nonoccupational (eg, sexual, needle-sharing) exposures to HIV have been published by the US Public Health Service, but they do not directly address nonoccupational HIV exposures unique to children (such as accidental exposure to human milk from a woman infected with HIV or a puncture wound from a discarded needle on a playground), and they do not provide antiretroviral drug information relevant to PEP in children. This clinical report reviews issues of potential exposure of children and adolescents to HIV and gives recommendations for PEP in those situations. The risk of HIV transmission from nonoccupational, nonperinatal exposure is generally low. Transmission risk is modified by factors related to the source and extent of exposure. Determination of the HIV infection status of the exposure source may not be possible, and data on transmission risk by exposure type may not exist. Except in the setting of perinatal transmission, no studies have demonstrated the safety and efficacy of postexposure use of antiretroviral drugs for the prevention of HIV transmission in nonoccupational settings. Antiretroviral therapy used for PEP is associated with significant toxicity. The decision to initiate prophylaxis needs to be made in consultation with the patient, the family, and a clinician with experience in treatment of persons with HIV infection. If instituted, therapy should be started as soon as possible after an exposure-no later than 72 hours-and continued for 28 days. Many clinicians would use 3 drugs for PEP regimens, although 2 drugs may be considered in certain circumstances. Instruction for avoiding secondary transmission should be given. Careful follow-up is needed for psychologic support, encouragement of medication adherence, toxicity monitoring, and serial HIV antibody testing.

Demineralization for inactivation of infectious retrovirus in systemically infected cortical bone: in vitro and in vivo experimental studies

BACKGROUND

Clinical and experimental studies have demonstrated viral transmission through the transplantation of fresh-frozen infected bone. While sterilization methods sufficient to inactivate the human immunodeficiency virus (HIV) have been shown to markedly alter osteoconductive and osteoinductive properties of bone allografts, the ability of a process for creating demineralized bone matrix to abrogate transmission of a retrovirus has not been investigated, to our knowledge. We hypothesized that a clinically accepted demineralization procedure would alter the nucleic acids of the feline leukemia virus (FeLV, a retrovirus with a structure and replication cycle similar to those of HIV), inactivating the virus in infected bone and rendering it noninfectious.

METHODS

Bone infected with FeLV was demineralized with a method employed for creating demineralized bone matrix powder. The effects of demineralization on cellular and (pro)viral nucleic acids were examined with use of gel electrophoresis and quantitative polymerase chain reaction, respectively. To compare the infectivity of the demineralized bone matrix with that of mineralized bone particles in cell cultures and in animals in which they had been implanted, we measured FeLV p27 antigen and (pro)viral nucleic acids as well as antiviral antibodies.

RESULTS

Demineralization of FeLV-infected bone appeared to inactivate the virus by degradation and fragmentation of the DNA, rendering it noninfectious in both in vitro and in vivo test systems. In contrast, untreated mineralized FeLV-infected bone contained intact nucleic acids and readily transmitted the virus in both test systems.

CONCLUSIONS

The demineralization process inactivated infectious retrovirus in infected cortical bone, thereby preventing disease transmission.

Management of Occupational and Nonoccupational Postexposure HIV Prophylaxis

The principles of managing patients with recent HIV exposure are similar whether the exposure occurs in an occupational or nonoccupational setting. For both settings, clinicians should assess the likelihood that HIV and other bloodborne viruses will be transmitted as a consequence of the exposure; advise the patient about the risks and benefits of treatment; choose an appropriate antiretroviral treatment regimen (if the decision is made to treat); screen for other illnesses that may complicate treatment or follow-up; counsel patients about the importance of adhering to treatment; promote safe-sex practices and methods to avoid future exposures; follow the patient for potential side effects of treatment; and provide follow-up care including repeat HIV testing for seroconversion, surveillance for primary HIV infection, and reinforcement of counseling messages.

Pharmacologic management of HIV/STD exposure

Emergency physicians must be familiar with management of occupational exposures to HIV-infected bodily fluids. Victims of sexual assault also may be exposed to HIV and other sexually transmitted diseases. The Centers for Disease Control and Prevention guidelines for the management of occupational exposures include recommendations for administration of combination antiretroviral therapy to reduce the risk for HIV transmission. Antiretroviral therapy also may be indicated for certain types of sexual exposure to HIV. Drugs that are used to treat sexually transmitted diseases also may be given to prevent infection after possible exposures. This article reviews current recommendations for prophylaxis against HIV and other sexually transmitted diseases and the scientific basis for these recommendations.

Effect of zidovudine on the primary cytolytic T-lymphocyte response and T-cell effector function

Azidothymidine (AZT) and other nucleoside analogues, used to treat AIDS, can cause severe clinical side effects and are suspected of suppressing immune cell proliferation and effector immune cell function. The purpose of the present study was to quantitatively measure the effects of AZT on cytotoxic T-lymphocyte (CTL) priming and to determine if the major histocompatibility complex-restricted CTL killing was affected by AZT exposure. For this purpose, we employed a murine alloantigen model and limiting-dilution analysis (LDA) to estimate cytotoxic effector cell frequencies of alloreactive splenocytes treated with drug during antigen sensitization. This noninfectious model was chosen to avoid analysis of a virus-compromised immune system. Exposure of splenocytes to therapeutic concentrations of AZT (2 to 10 microM) caused a two- to threefold dose-dependent reduction in CLT precursor frequency. This reduction was caused by decreased proliferation of alloantigen-specific CTLs rather than loss of function, because full cytolytic function could be restored by adjusting the AZT-treated effector/target cell ratios to that of untreated cells. In addition, when AZT was added to the assay system at various times during antigen sensitization there was a time-related loss of the suppressive effect on the generation of cytolytic effector function, suggesting that functional CTLs are not affected by even high doses of AZT. Taken together, the data indicate that the reduction of CTL function associated with AZT treatment is due to a quantitative decrease of effector cell precursor frequency rather than to direct drug cytotoxicity or interference with mediation of cytolysis. Furthermore, antigen-naive immune cells were most sensitive to this effect during the first few days following antigen encounter.

Emergency department management of blood and body fluid exposures

Exposure to blood and body fluids that may be contaminated with infectious agents is a common occupational hazard for health care workers. Health care workers in the emergency department or out-of-hospital setting are at especially high risk for exposure to blood or body fluids. Nonemergency health care workers are frequently referred to hospital EDs for immediate treatment of occupation exposures. A series of recommendations by the Centers for Disease Control and Prevention evolved over the past decade, and changes are expected to continue. This state-of the-art article reviews current recommendations for management of persons exposed to blood or body fluids and discusses the scientific basis for recommendations regarding hepatitis B virus, hepatitis C virus, and HIV.

Post-exposure prophylaxis for HIV

The advent of preventive treatment for HIV highlights the urgent need for basic, clinical, and epidemiologic research targeting the pathogenesis and prevention of cutaneous and mucosal infection. In addition, the impact of HIV prophylaxis on the frequency of risk behaviors and antiretroviral drug resistance, especially in cities with high HIV prevalence, must be evaluated. In our view, the available data (albeit not definitive) are strong enough to support the provision of post-exposure prophylaxis in select cases of sexual and injection drug use exposure in addition to occupational exposures. However, post-exposure treatment should be used judiciously and only in the context of a comprehensive prevention program. Ultimately, primary exposure prevention, whether in health care facilities or in the community, is the most important strategy to prevent AIDS.

Adoptive immunotherapy of feline leukemia virus infection using autologous lymph node lymphocytes

Adoptive immunotherapy using autologous cells expanded ex vivo from lymph nodes was examined in cats infected with the retrovirus feline leukemia virus (FeLV). Cells were obtained from popliteal lymph nodes from 18 FeLV-antigen-positive cats without complications; a mean of 6.2 x 10(7) cells were obtained. Lymph node cells were cultured with 600 IU/ml interleukin-2 (IL-2) for 7 days. Cells expanded 0.8- to 11-fold (mean, 2.7; median, 2.4); were 80% +/- 8.0% CD3+, 29% +/- 8.1% CD4+, and 41% +/- 7.0% CD8+, and exhibited cytolytic activity against FeLV-transformed FL74 cells. Sixteen cats received a single intravenous infusion of 0.13 to 3.9 x 10(8) cells. Cell infusion was well tolerated; fever developed approximately 1 hour postinfusion. Clinical activity, antiviral activity, or both was observed in 10 cats. Nine cats had clinical responses with improvement in weight, activity, appearance, or a combination of these that began 2 to 4 weeks after cell infusion and that lasted for up to 13 or more months. FeLV antigen became undetectable in 4 cats. These results indicate that adoptive immunotherapy using autologous lymph node cells, activated and expanded ex vivo in short-term cultures with low concentrations of IL-2, can modulate the course of a retroviral infection.

Evidence that high-dosage zidovudine at time of retrovirus exposure reduces antiviral efficacy

The antiviral efficacy of prophylactic 3'-azido-3'-deoxythymidine (AZT) therapy administered by continuous infusion or intermittent injection was compared in pediatric cats infected with feline leukemia virus. A 4-week treatment regimen of AZT was initiated at -48, 8, or 96 h postinfection (p.i.). For AZT therapy begun at -48 h p.i., significant efficacy was attained when therapy was given by continuous infusion but not by intermittent injection. However, when AZT therapy was delayed until 96 h p.i., both continuous infusion and intermittent injection gave complete protection. The results suggest that intermittent AZT administration is less efficacious than continuous infusion. Higher peak AZT concentrations in plasma associated with intermittent injection compared with those associated with continuous infusion may be immunotoxic, thus reducing the drug-induced vaccine effect. Furthermore, AZT toxicity seemed to be restricted to a window of sensitivity close to the time of virus challenge because delaying the start of AZT therapy until 96 h p.i. was highly efficacious, regardless of the method of administration.

Side effects of AZT prophylaxis after occupational exposure to HIV-infected blood

It was the objective of this study to document and evaluate AZT-induced short-term toxicity in healthy individuals. The study was designed as a longitudinal monocentric side-effect monitoring study with prospective data collection. It was carried out at the Cologne University Hospital. The study population comprised health care workers who were taking AZT prophylaxis after accidental exposure to HIV-infected blood. Fourteen individuals were included into the study; seven of them discontinued treatment prematurely, five due to severe subjective symptoms. In case of one worker AZT had to be stopped due to severe neutropenia (800 cells/microliters) with signs of upper respiratory tract infection. Four of 11 individuals taking AZT for at least 4 weeks developed neutropenia (2 WHO I, 1 WHO II, 1 WHO III). All other laboratory parameters stayed within normal range. In particular, no anemia was observed.

IN CONCLUSION

Compared with other studies more neutropenias are observed. Due to side effects 50% of the workers discontinued AZT administration prematurely. The data presented herein show that AZT causes considerable side effects which must be weighed against the potential protective antiviral effect.

Differential antiviral activities and intracellular metabolism of 3'-azido-3'-deoxythymidine and 2',3'-dideoxyinosine in human cells

Dideoxynucleosides such as 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxyinosine (ddI) can effectively inhibit the replication of human immunodeficiency virus (HIV) in T lymphoid cells. There is evidence that HIV can infect and replicate in other cells including monocytoid cells and macrophages. The present study compared the antiretroviral activities of ddI and AZT in three lineages of human cells, i.e., MOLT4 (T lymphocytoid, CD4+), U937 (monocytoid, CD4+), and HT1080 (fibroblastoid, CD4-) cells. Feline leukemia virus, a retrovirus that causes immunodeficiency in cats, was used to infect the cells. The drug concentrations needed to reduce the viral p27 antigen titers in cell lysates by 50% (IC50s) were determined. The data show that AZT and ddI inhibited viral replication in all three cell lines. The IC50s of AZT were 0.02, 1.75, and 2.31 microM in MOLT4, HT1080, and U937 cells, respectively. For ddI, the IC50s were 4.31, 9.52, and 43.5 microM, respectively. These data indicate differential antiviral activities of ddI and AZT in the different cells with the following rank order of drug sensitivity: MOLT4 > HT1080 > U937. A study of the intracellular metabolism of [3H]AZT and [3H]ddI shows that the antiretroviral activities of AZT and ddI in the three cell lines correlated with the levels of their intracellular triphosphate metabolites.

Chemoprevention of retroviral infection: success is determined by virus inoculum strength and cellular immunity

We demonstrated earlier that post-exposure prophylaxis with 3'-azido-3'-deoxythymidine (AZT, zidovudine) or with AZT + interferon-alpha (IFN-alpha) prevented viremia and disease in BALB/c mice inoculated with Rauscher murine leukemia virus (RLV). After the 20-day treatment course, most animals were resistant to rechallenge with live virus. Adoptive transfer of T cells from such resistant but not from normal mice into naive recipients provided full protection against virus challenge. From these experiments, we concluded that post-exposure chemoprophylaxis restricted virus replication and allowed the animals to form protective, long-lasting cellular immune responses. Here, the role for cellular immunity during antiviral chemoprophylaxis was tested by comparing treatment success in normal BALB/c mice and in their nude, athymic counterparts. Both were inoculated with equal doses of RLV (10(4) plaque-forming units, pfu). Single-agent AZT or combination therapy with AZT + IFN-alpha, started before or after RLV inoculation, prevented viremia in all normal but not in most nude mice. A significant number of nude mice were completely protected by chemoprevention only when given a 10 times lower virus dose. When normal mice were injected with a 10 times higher virus dose (10(5) pfu), complete protection by chemoprevention was lost. These results demonstrate that the success of chemoprevention depends critically on the virus inoculum. The differential success of chemoprevention in normal and T-cell-deficient mice implies that effective cellular immunity plays an important role in protecting virus-exposed animals against viremia and disease.

Prophylaxis of HIV infection following occupational exposure

OBJECTIVE

To review the risk of HIV infection following occupational exposure, the theoretical basis for chemoprophylaxis, investigative experience with chemoprophylaxis in animals and humans, and the economic aspects of postexposure chemoprophylaxis.

DATA SOURCES

English-language articles and conference proceedings pertaining to the risk of occupational HIV infection and to postexposure chemoprophylaxis.

STUDY SELECTION

Studies evaluating chemoprophylaxis of HIV infection following occupational exposure were selected for review. Abstracts reporting ongoing clinical trials were also included.

DATA EXTRACTION

In vitro studies are discussed to provide the immunologic rationale for chemoprophylaxis. Animal studies examining the efficacy of chemoprophylaxis in preventing non-HIV retroviral infection are reviewed, and their applicability to human HIV infection is critically evaluated. Human studies and case reports describing attempts at chemoprophylaxis of HIV infection following occupational exposure are discussed.

DATA SYNTHESIS

Chemoprophylaxis of HIV infection following occupational exposure has focused on the use of zidovudine (ZDV) because it was previously the only antiretroviral agent approved for treating HIV infection. Animal models of retroviral infection provide conflicting data regarding the efficacy of ZDV chemoprophylaxis, and there are important questions about the applicability of animal data to human HIV infection because of differences in natural histories of non-HIV retroviral infections, inoculum size, dosing of ZDV, and routes of infection. Human surveillance studies are thus far inadequate to determine the efficacy of ZDV prophylaxis because of the very low HIV seroconversion rates following occupational exposure. ZDV is well tolerated during short-term administration in people without HIV infection, but long-term safety is unknown. In addition, the true cost-benefit ratio of ZDV chemoprophylaxis is uncertain.

CONCLUSIONS

Current data from in vitro, animal, and human studies are inadequate to define the appropriate role of ZDV in preventing HIV infection following occupational exposure. Limited toxicity data and the high cost of treatment must be weighed against the theoretical benefits of ZDV use in this setting. The decision to employ ZDV for postexposure prophylaxis must ultimately be based on existing institutional policies, the attitude of the responsible physician regarding such practice, and/or the desires of the exposed healthcare worker after being properly informed of potential risks and benefits.