Comparative pharmacokinetics of two prodrugs of zidovudine in rabbits: enhanced levels of zidovudine in brain tissue

Polymeric nanotheranostics for solid tumor management: Recent developments and global regulatory landscape

Polymeric nanotheranostics have emerged as promising vehicles for diagnosis‐cum‐targeted therapy in solid tumors, offering precise delivery of therapeutic agents at the site of solid tumors and minimizing systemic side effects. This article summarizes the latest developments in using polymeric nanoparticles for specific treatment strategies in solid tumors. It explores the various methods these nanoparticles utilize for targeted medication delivery. This includes passive targeting through the amplified permeability and retention effect, active targeting via interactions between ligands and receptors, and stimuli‐responsive release mechanisms such as pH, temperature, and enzymatic triggers. Furthermore, we highlight recent developments in stimuli‐responsive polymeric nanoparticles, which enable controlled drug release in response to specific cues in the tumor microenvironment, thus enhancing therapeutic efficacy. Also, we focus on the theranostic polymeric nanoparticles, which are used for diagnosing and treating solid tumors. We discuss critical regulatory considerations and the regulatory bodies of different countries that regulate nanomedicines' safety, efficacy, quality, and manufacturing processes. Overall, this review provides insights into the latest innovations in polymeric nanoparticles for targeted therapy in solid tumors, elucidating their mechanisms of action, stimuli‐responsive properties, and regulatory pathways, which collectively contribute to developing effective and safe nanomedicines for cancer treatment.

Targeting Transporters for Drug Delivery to the Brain: Can We Do Better?

Limited drug delivery to the brain is one of the major reasons for high failure rates of central nervous system (CNS) drug candidates. The blood–brain barrier (BBB) with its tight junctions, membrane transporters, receptors and metabolizing enzymes is a main player in drug delivery to the brain, restricting the entrance of the drugs and other xenobiotics. Current knowledge about the uptake transporters expressed at the BBB and brain parenchymal cells has been used for delivery of CNS drugs to the brain via targeting transporters. Although many transporter-utilizing (pro)drugs and nanocarriers have been developed to improve the uptake of drugs to the brain, their success rate of translation from preclinical development to humans is negligible. In the present review, we provide a systematic summary of the current progress in development of transporter-utilizing (pro)drugs and nanocarriers for delivery of drugs to the brain. In addition, we applied CNS pharmacokinetic concepts for evaluation of the limitations and gaps in investigation of the developed transporter-utilizing (pro)drugs and nanocarriers. Finally, we give recommendations for a rational development of transporter-utilizing drug delivery systems targeting the brain based on CNS pharmacokinetic principles.

Antiretroviral bioanalysis methods of tissues and body biofluids

Research in the many areas of HIV treatment, eradication and prevention has necessitated measurement of antiretroviral (ARV) concentrations in nontraditional specimen types. To determine the knowledgebase of critical details for accurate bioanalysis, a review of the literature was performed and summarized. Bioanalytical assays for 31 ARVs, including metabolites, were identified in 205 publications measuring various tissues and biofluids. 18 and 30% of tissue or biofluid methods, respectively, analyzed more than one specimen type; 35-37% of the tissue or biofluid methods quantitated more than one ARV. 20 and 76% of tissue or biofluid methods, respectively, were used for the analysis of human specimens. HPLC methods with UV detection predominated, but chronologically MS detection began to surpass. 40% of the assays provided complete intra- and inter-assay validation data, but only 9% of publications provided any stability data with even less for the prevalent ARV in treatments.

HIV pharmacology: barriers to the eradication of HIV from the CNS

Total eradication of HIV-1 is not yet achievable, in part because reservoirs of latent HIV-1 can develop within lymphoid tissue, the testes, and the central nervous system (CNS). The presence of HIV-1 in the CNS is clinically significant because of its association with the development of HIV dementia, which occurs in up to one fifth of untreated patients. This review summarizes current theory regarding HIV-1 infection within the CNS, describes physiologic and pharmacologic factors limiting CNS penetration of antiretroviral drugs used to treat HIV-1 infection, and reviews current treatment of CNS HIV-1 infection and HIV encephalopathy.

Synthesis, in vitro anticancer evaluation, and interference with cell cycle progression of N-phosphoamino acid esters of zidovudine and stavudine

A series of N-diisopropylphosphoryl (DIPP) L-amino acid ester prodrugs of zidovudine (AZT) (3a-3e) and stavudine (d4T) (4a-4e) has been prepared. The activity of these compounds against MCF-7 cells (human pleural effusion breast adenocarcinoma cell line) and K562 cells (human chronic myeloid leukemia (CML) cell line) was evaluated. In difference from that of AZT amino acid phosphoramidates, the alophatic amino acid esters of AZT were found to be more cytotoxic than the aromatic analogues toward MCF-7 cell. Two DIPP-L-amino acid esters of d4T 4b (CC50 = 83 microM) and 4c (CC50 = 182 microM) were found to be more cytotoxic than the parent drug toward K562 cells. MCF-7 and K562 cell cycle disturbance was investigated showing detectable blockade in the S phase when exposed to biologically active AZT, 3a, 3b, 3c, 4b and 4c, indicating that they inhibit cell growth by blocking cell cycle progression. Together with previous reports, present findings suggest that anti-breast cancer activity of AZT may be due to hamper DNA synthesis.

Pharmacokinetics of novel dipeptide ester prodrugs of acyclovir after oral administration: intestinal absorption and liver metabolism

The amino acid prodrug of acyclovir (ACV), valacyclovir (VACV), is an effective antiherpetic drug. Systemic availability of ACV in humans is 3 to 5 times higher after oral administration of VACV. Enhanced bioavailability of VACV has been attributed to its carrier-mediated intestinal absorption via hPEPT1 peptide transporter followed by rapid and complete conversion to ACV. An earlier report suggested that the dipeptide ester prodrugs of ACV possess high affinity toward the intestinal oligopeptide transporter hPEPT1 and therefore seem to be promising candidates in the treatment of oral herpes virus infections. In the present study, we have examined the bioavailability of a series of dipeptide prodrugs of ACV after oral administration in Sprague-Dawley rats with cannulated jugular and portal veins. The area under plasma-concentration time curves expressed as minutes microgram milliliter(-1) for total concentration of VACV (208.4 +/- 41.2), and the dipeptide prodrugs Gly-Val-ACV (GVACV) (416.1 +/- 140.9), Val-Val-ACV (VVACV) (147.7 +/- 89.3), and Val-Tyr-ACV (VYACV) (180.7 +/- 81.2) were significantly higher than that of ACV (21.2 +/- 5.2) upon intestinal absorption. Interestingly, the bioavailability of ACV after administration of GVACV was approximately 2-fold higher than VACV. There was significant metabolism by hepatic first pass effect of the dipeptide prodrugs as evident by the higher levels of ACV obtained after systemic absorption compared with intestinal absorption of GVACV and VVACV. The dipeptide prodrugs of ACV exhibited higher systemic availability of regenerated ACV upon oral administration and thus seem to be promising drug candidates in treatment of genital herpes infections.

Mechanism of corneal permeation of L-valyl ester of acyclovir: targeting the oligopeptide transporter on the rabbit cornea

PURPOSE

To delineate mechanisms associated with the corneal transport of a L-valine prodrug of an antiviral agent, acyclovir.

METHOD

The permeability and enzymatic hydrolysis of L-Val-ACV were evaluated using freshly excised rabbit cornea. Transport mechanism across rabbit cornea was investigated through a competitive inhibition study of L-Val-ACV with other substrates of human peptide transporter (hPepT1).

RESULTS

L-Valyl ester of Acyclovir (L-Val-ACV) was approximately threefold more permeable across the intact rabbit cornea than acyclovir (ACV). Dipeptides, beta-lactam antibiotics, and angiotensin converting enzyme (ACE) inhibitors, strongly inhibited the transport of L-Val-ACV indicating that a carrier mediated transport system specific for peptides is primarily responsible for the corneal permeation of L-Val-ACV. L-Val-ACV transport was found to be saturable (Km = 2.26 +/- 0.34 mM, Jmax = 1.087 +/- 0.05 nmoles cm(-2) min(-1)), energy and pH dependent. CONCLUSIONS Functional evidence of an oligopeptide transport system present on the rabbit cornea has been established. The peptide transporter on the corneal epithelium may be targeted to improve the ocular bioavailability of poorly absorbed drugs.

Novel dipeptide prodrugs of acyclovir for ocular herpes infections: Bioreversion, antiviral activity and transport across rabbit cornea

PURPOSE

A series of dipeptide prodrugs of antiviral nucleoside acyclovir (ACV) were designed to target the oligopeptide transporter on the cornea with an aim of improving the ocular bioavailability and therapeutic activity of ACV.

METHODS

Aqueous stability, ocular bioreversion kinetics in various tissues, in vitro antiviral activity, cell proliferation assay and corneal transport characteristics of the dipeptide prodrugs were studied. Results. ACV dipeptide prodrugs were found to be more stable at pH 5.6 in comparison to L-Val-ACV, an amino acid prodrug of ACV. The prodrugs exhibited higher solubility than ACV. Val-Val-ACV and Val-Tyr-ACV were found to have excellent antiviral activity against herpes simplex virus-1 (HSV-1). All the dipeptide prodrugs exhibited lower cytotoxicity as compared to currently approved anti-HSV agent, trifluorothymidine (TFT). Transport of [(3)H] Val-ACV was inhibited significantly in the presence of the dipeptide prodrugs of ACV. Corneal permeabilities of all the ACV dipeptide prodrugs were observed to be higher than ACV possibly due to recognition of the prodrugs by the oligopeptide transporter on the cornea.

CONCLUSIONS

The dipeptide prodrugs were found to be more permeable than the parent drug, ACV. More permeable, less cytotoxic ACV dipeptide prodrugs exhibited excellent chemical stability and antiviral activity against herpes simplex virus thereby rendering these lead compounds promising drug candidates against herpes virus infections.

Interactions of the dipeptide ester prodrugs of acyclovir with the intestinal oligopeptide transporter: competitive inhibition of glycylsarcosine transport in human intestinal cell line-Caco-2

The oligopeptide transporter may be exploited to enhance the absorption of drugs by synthesizing their dipeptide ester prodrugs, which may be recognized as its substrates. Various dipeptide esters of acyclovir (ACV), an antiviral nucleoside analog, were synthesized. Enzymatic hydrolysis and affinity of the prodrugs toward the human intestinal peptide transporter hPEPT1 were studied using the human intestinal Caco-2 cell line. Affinity studies were performed by inhibiting the uptake of [(3)H]glycylsarcosine by the prodrugs. The uptake of glycylsarcosine was found to be saturable at higher concentrations and was competitively inhibited by the prodrugs of ACV. All prodrugs except Tyr-Gly-ACV demonstrated a higher affinity (1.41-4.96 mM) toward hPEPT1 than cephalexin (8.19 +/- 2.12 mM), which was used as a positive control. Two prodrugs, Gly-Val-ACV and Val-Val-ACV, showed comparable affinity to Val-ACV, an amino acid prodrug of ACV recognized by PEPT1/PEPT2. The permeability of Gly-Val-ACV (2.99 +/- 0.59 x 10(-6) cm/s) across Caco-2 was comparable with that of Val-ACV (3.01 +/- 0.21 x 10(-6) cm/s) and was significantly inhibited (63%) in presence of glycylsarcosine. The transport of GVACV across Caco-2 was saturable at higher concentrations, and the parameters were calculated as K(m) 3.16 +/- 0.31 mM and V(max) 0.014 +/- 0.00058 nmol cm(-2) min(-1). Overall, the results suggest that the dipeptide prodrugs of ACV have a high affinity toward the intestinal oligopeptide transporter hPEPT1 and therefore seem to be promising candidates in the treatment of ocular and oral herpesvirus infections, because cornea and intestinal epithelia seem to express the oligopeptide transporters.

Intravitreal, retinal, and central nervous system foscarnet concentrations after rapid intravenous administration to rabbits

Retinal, vitreous humor, brain, and cerebrospinal fluid (CSF) foscarnet levels were measured by high-performance liquid chromatography after administration of an intravenous dose of 120 mg/kg of body weight to 32 pigmented rabbits. A pharmacokinetic analysis was done using a two-compartment model. The penetration ratios, defined as ratios of retinal, vitreous humor, brain, and CSF areas under the concentration-time curve from 0 to 2 h were 110% +/- 1%, 12.3% +/- 0.7%, 118% +/- 1%, and 20.2% +/- 2.2%, respectively. These results suggest a good penetration of foscarnet into the retinal and brain tissues, reaching higher concentrations than those estimated from vitreous humor and CSF levels.

Recent advances in retrometabolic design approaches

The retrometabolic drug design approaches simultaneously incorporate structure activity (SAR) and structure metabolism (SMR) relationships in the design process. Two major approaches were developed, the chemical delivery systems (CDS), which allow chemical-enzymatic targeting of drugs via strategic sequential enzymatic activation of the inactive CDSs. On the opposite end of the retrometabolic design loop are the soft drugs (SD), which are designed to have highly improved therapeutic indeces by controlling their metabolism, after they achieve their therapeutic role. One of the most successful SD class is the 'inactive metabolite approach', where the design starts from an inactive metabolite of a drug. Its strategic manipulation yields an isosteric/isoelectronic drug analog, which is enzymatically deactivated to the very inactive metabolite at the desired compartment and with controlled rate. Overall, retrometabolic approaches represent a complex collection of chemical-enzymatic means for the design of safer drugs and for their controlled release. Most recent advances involve FDA approval of a soft steroid, as well as the first successful brain targeting of various neuropeptides and their brain-targeted analogs.

Investigation of distribution, transport and uptake of anti-HIV drugs to the central nervous system

The distribution of currently available anti-HIV drugs into the CNS is reviewed with a focus on transport mechanisms. Among these drugs, nucleoside analogs are most well studied for their CNS distribution. The average reported values of the CSF/plasma steady-state concentration or corresponding AUC ratios are 0.23 (AZT), 0.06 (ddI), 0.04 (ddC), 0.49 (d4T), and 0.08 (3TC). Active efflux transport out of the CNS appears to be a predominant mechanism limiting nucleoside access to the CNS, although poor penetration may contribute to some extent for some polar nucleosides. The nature of the efflux pump for these drugs is speculated to be MRP-like transporter(s) in blood-brain and blood-CSF barriers. For non-nucleoside and protease inhibitors, much research remains to be done on the extent, time course, and mechanisms of their CNS distribution. The CNS penetration of some protease inhibitors is restricted by P-glycoprotein. A better understanding of transport mechanisms of anti-HIV drugs in the CNS is essential to develop approaches to enhance CNS delivery of available drugs and to identify new drugs less subject to active efflux transporter(s) in the CNS.

Concepts for the design of anti-HIV nucleoside prodrugs for treating cephalic HIV infection

The life cycle of HIV involves nine sequential stages. Of these, the reverse transcription (RT) process is a prime target for drug therapy, using both nucleoside and non-nucleoside inhibitors of RT. There are currently five marketed 2',3'-dideoxynucleoside RT inhibitors, but there is need for drugs with improved therapeutic efficacy, decreased development of resistance and broader spectrum to treat resistant strains.One approach to improve RT inhibitors is through chemical derivatization using metabolically-cleavable linkages that permit timely regeneration of the active nucleoside inside the body at the site of infection (prodrug formation). Four classes of prodrugs are now reviewed: 2',3'-dideoxynucleoside masked phosphates, 5'-O-carboxylic acid esters of 2',3'-dideoxynucleosides, 2',3'-dideoxycytidine N(4)-[(dialkylamino)methylene] prodrugs and 5-halo-6-alkoxy(azido or hydroxy)-5,6-dihydro 2',3'-dideoxynucleosides. Mutually-masking dual action (MMDA) prodrugs that release a nucleoside RT inhibitor and an abnormal N-myristoyl transferase substrate are presented as a special class of anti-HIV prodrugs that have the potential to interact with the life cycle of the virus at two distinct stages.

Recent advances in the brain targeting of neuropharmaceuticals by chemical delivery systems

Brain-targeted chemical delivery systems represent a general and systematic method that can provide localized and sustained release for a variety of therapeutic agents including neuropeptides. By using a sequential metabolism approach, they exploit the specific trafficking properties of the blood-brain barrier and provide site-specific or site-enhanced delivery. After a brief description of the design principles, the present article reviews a number of specific delivery examples (zidovudine, ganciclovir, lomustine benzylpenicillin, estradiol, enkephalin, TRH, kyotorphin), together with representative synthetic routes, physicochemical properties, metabolic pathways, and pharmacological data. A reevaluated correlation for more than 60 drugs between previously published in vivo cerebrovascular permeability data and octanol/water partition coefficients is also included since it may be useful in characterizing the properties of the blood-brain barrier, including active transport by P-glycoprotein.

Cellular uptake mechanism of amino acid ester prodrugs in Caco-2/hPEPT1 cells overexpressing a human peptide transporter

PURPOSE

This study characterized the cellular uptake mechanism and hydrolysis of the amino acid ester prodrugs of nucleoside antiviral drugs in the transiently transfected Caco-2 cells overexpressing a human intestinal peptide transporter, hPEPT1 (Caco-2/hPEPT1 cells).

METHODS

Amino acid ester prodrugs of acyclovir and AZT were synthesized and their apical membrane permeability and hydrolysis were evaluated in Caco-2/hPEPT1 cells. The cellular uptake mechanism of prodrugs was investigated through the competitive inhibition study in Caco-2/hPEPT1 cells.

RESULTS

L-Valyl ester of acyclovir (L-Val-ACV) was approximately ten fold more permeable across the apical membrane than acyclovir and four times more permeable than D-valyl ester of acyclovir (D-Val-ACV). Correspondingly, L-valyl ester of AZT (L- Val-AZT) exhibited three fold higher cellular uptake than AZT. Therefore, amino acid ester prodrugs significantly increased the cellular uptake of the parent drugs and exhibited the D,L-stereoselectivity. Furthermore, prodrugs were rapidly hydrolyzed to the parent drugs by the intracellular hydrolysis, following the apical membrane transport. In the inhibition studies, cephalexin and small dipeptides strongly inhibited the cellular uptake of L-Val-ACV while L-valine had no effect, indicating that the peptide transporter is primarily responsible for the apical membrane transport of L-Val-ACV. In addition, the cellular uptake of L-Val-ACV was five times higher in Caco-2/hPEPT1 cells than the uptake in the untransfected Caco-2 cells, implying the cellular uptake of L-Val-ACV was related to the enhancement of the peptide transport activity in Caco-2/hPEPT1 cells.

CONCLUSIONS

Caco-2/hPEPT1 system is an efficient in vitro model for the uptake study of peptidyl derivatives. Amino acid ester prodrugs significantly improved the cellular uptake of the parent drugs via peptide transport mechanism and were rapidly converted to the active parent drugs by the intracellular hydrolysis.

Cerebrospinal-fluid HIV-1 RNA and drug concentrations after treatment with lamivudine plus zidovudine or stavudine

BACKGROUND

Treatment and prevention of HIV-1-related central-nervous-system disease may be dependent on penetration of antiretroviral drugs into the central nervous system. Few data are available about cerebrospinal-fluid penetration and concomitant changes of HIV-1-RNA concentrations during treatment with antiretroviral agents. We investigated these effects in HIV-1-infected people.

METHODS

28 antiretroviral-naive individuals with CD4 cell counts of 200/microL or more and plasma HIV-1-RNA concentrations of 10,000 or more copies/mL who were free of neurological symptoms were randomly assigned lamivudine plus either stavudine (n = 17) or zidovudine (n = 11). We did lumbar punctures on 28 individuals before and 22 individuals after 12 weeks of treatment to assess HIV-1-RNA and drug concentrations in cerebrospinal fluid.

FINDINGS

All 28 individuals had detectable HIV-1-RNA concentrations in the cerebrospinal fluid (median 4.64 log10 copies/mL and 4.20 log10 copies/mL in the lamivudine plus zidovudine and lamivudine plus stavudine groups, respectively). There was no correlation between plasma and cerebrospinal-fluid HIV-1-RNA concentrations (r = 0.18, p = 0.35). After 12 weeks of treatment none of the individuals had detectable HIV-1-RNA concentrations in the cerebrospinal fluid. The highest drug concentration in the cerebrospinal fluid was for lamivudine followed by stavudine and zidovudine. Concentrations were consistent over time, unlike plasma concentrations. Therefore, we found time-dependent cerebrospinal-fluid to plasma drug-penetration ratios, which were highest for zidovudine followed by stavudine and lamivudine.

INTERPRETATION

The two drug combinations were equally effective in the decrease of cerebrospinal fluid HIV-1-RNA concentrations. All drugs penetrated the cerebrospinal fluid. Antiretroviral drugs other than zidovudine might be useful in the prevention of AIDS dementia complex.

Evaluation of a brain-targeting zidovudine chemical delivery system in dogs

AIDS encephalopathy is an insidious complication of human immunodeficiency virus infection which is difficult to treat because of the poor uptake of many potentially useful antiretroviral drugs through the blood-brain barrier. A chemical delivery system (CDS) for zidovudine (AZT) based on redox trapping within the brain has been prepared and tested in several animal models to circumvent this limitation. The behavior of the AZT-CDS in the dog was considered. Parenteral administration of AZT resulted in rapid systemic elimination and poor uptake by the central nervous system. Ratios of the area under the concentration-time curve of AZT for cerebrospinal fluid to that for blood were 0.32, and ratios of the area under the concentration-time curve of AZT for brain to that for blood were approximately 0.25. Administration of an aqueous formulation of the AZT-CDS resulted in rapid tissue uptake and conversion of the CDS to the corresponding quaternary salt with the subsequent production of AZT. Delivered in this way, the levels of AZT in brain were 1.75- to 3.3-fold higher than those associated with conventional AZT administration. In addition, the levels of AZT in blood were 46% lower than those associated with AZT administration. The higher concentrations in brain and lower concentration in blood combined to significantly increase the ratio of the concentration of AZT in the brain to that in blood after AZT-CDS administration compared to that after AZT dosing.

Comparative brain exposure to (-)-carbovir after (-)-carbovir or (-)-6-aminocarbovir intravenous infusion in rats

PURPOSE

Evaluate the ability of (-)-6-aminocarbovir ((-)-6AC) to improve the CNS exposure to (-)-carbovir ((-)-CBV).

METHODS

Activation of (-)-6AC in vitro was assessed by incubations of rat brain tissue homogenates. The in vivo brain exposure to (-)-CBV was then examined in rats after iv infusions of either (-)-CBV (n = 4) or (-)-6AC (n = 5). The drugs were infused to steady-state via the jugular vein. At the end of the infusion, a bolus of [3H]inulin was injected via the femoral vein in order to obtain an estimate of the brain vascular space.

RESULTS

(-)-6AC was converted to (-)-CBV by incubations of rat brain tissue homogenates. After iv infusion of (-)-CBV, the brain/blood concentration ratio of (-)-CBV was 0.032 +/- 0.009. The brain/blood concentration ratio of (-)-CBV after iv infusion of (-)-6AC was 0.080 +/- 0.020.

CONCLUSIONS

(-)-6AC improved the brain delivery of (-)-CBV, although the absolute exposure of the brain tissue to (-)-CBV was still quite low.

Enhanced delivery of ganciclovir to the brain through the use of redox targeting

Enhanced delivery of ganciclovir to the brain was demonstrated by a redox-based chemical delivery system. A ganciclovir monoester in which a 1-methyl-1,4-dihydronicotinate was covalently attached to one of the hydroxymethyl functions was prepared. The stability of the ganciclovir chemical delivery system (DHPG-CDS) was evaluated in aqueous buffers and organ homogenates. In vivo distribution studies in the rat indicated that while ganciclovir poorly penetrated into the central nervous system and was rapidly eliminated, DHPG-CDS provided for therapeutically relevant (2.7 microM) and sustained levels of the parent compound through 6 h. An analysis of the area under the concentration curve indicated that the chemical delivery system delivered five times more ganciclovir than that of the parent drug. The high levels in the brain and reduced levels in the blood gave a brain-to-blood drug concentration ratio of 2.54 for ganciclovir when delivered by the chemical delivery system, compared to a ratio of 0.063 when the parent drug was administered. These data suggest that DHPG-CDS could be a useful adjunct for the treatment of cytomegalovirus encephalitis.

Zidovudine. An update of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy

Zidovudine remains the mainstay in the treatment of patients infected with human immunodeficiency virus (HIV). The drug delays disease progression to acquired immunodeficiency syndrome (AIDS) and to AIDS-related complex (ARC), reduces opportunistic infections, and increases survival in patients with advanced HIV infection. There is evidence to suggest that zidovudine also delays disease progression in patients with mild symptomatic disease. Although one study has shown zidovudine to have no significant beneficial effects on survival or disease progression in patients with asymptomatic HIV infection, several other studies have shown zidovudine to delay disease progression in this patient group. Results from related ongoing studies are awaited with interest. Zidovudine reduces the incidence of AIDS dementia complex (ADC) and appears to prolong survival in these patients, and improves other neurological complications of HIV infection. The drug also appears to enhance the efficacy of interferon-alpha in patients with Kaposi's sarcoma. Although zidovudine is widely used as postexposure prophylaxis following accidental exposure to HIV, its efficacy in preventing seroconversion is unclear. Whether zidovudine prevents vertical transmission also remains to be determined. The overall efficacy of zidovudine in the treatment of children with HIV infection appears similar to that in adults despite more rapid disease progression in younger patients. Zidovudine-resistant isolates can emerge as early as after 2 months' therapy, and primary infection with zidovudine-resistant strains has been documented. Both zidovudine resistance and the syncytium-inducing HIV phenotype appear to be associated with poor clinical outcome. However, zidovudine resistance may revert on drug withdrawal or switching to an alternative therapy. Zidovudine-associated haematotoxicity may be dose-limiting. Nonhaematological adverse events associated with zidovudine therapy are generally mild and usually resolve spontaneously. Dosages of approximately 500 to 600 mg/day appear to be at least as effective as dosages of 1200 to 1500 mg/day and are better tolerated in patients with less advanced disease. However, optimal dosage are unclear. Despite beneficial effects, zidovudine monotherapy is not curative. There is evidence to suggest that the concomitant administration of zidovudine with didanosine or zalcitabine is effective in patients with HIV disease progression despite receiving zidovudine monotherapy, and there is some evidence that concomitant zidovudine plus didanosine therapy is more effective than alternating monotherapy. However, results from studies of combination therapy in asymptomatic patients, and from comparative combination therapy studies are awaited. Cotherapy with agents that augment haematopoiesis allows the continuation of therapeutic zidovudine dosages.(ABSTRACT TRUNCATED AT 400 WORDS)