Treatment of AIDS and AIDS-related complex with 2',3'-dideoxyinosine given once daily

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

Didanosine Therapy in Patients Intolerant of or Failing Zidovudine Therapy

OBJECTIVE:

To examine the effect of didanosine (2′,3′-dideoxyinosine, ddI) on surrogate markers of HIV infection (CD4+ lymphocyte count, p24 antigen) and to evaluate the incidence of adverse effects from ddI.

DESIGN:

This study was performed as a retrospective chart review of patients who were enrolled in Bristol-Myers Squibb's expanded-access program for ddI.

SETTING:

Patient records were obtained from primary care physicians' offices.

PATIENTS:

Twenty-five HIV-infected patients diagnosed with AIDS or AIDS-related complex (ARC) who were intolerant of zidovudine (ZDV) therapy or deteriorating clinically despite ZDV therapy and were eligible for inclusion in the ddI expanded-access program.

INTERVENTION:

ddI was administered orally in a citrate-phosphate buffer every 12 hours. Patients were followed by their private physician on a monthly basis.

MAIN OUTCOME MEASURES:

Laboratory analysis at each month included CD4+ lymphocyte count, hemoglobin, hematocrit, serum amylase, uric acid, serum triglycerides, and p24 antigen. Mean CD4+ cell count, serum amylase, hemoglobin, and uric acid at each month during ddI therapy were compared with baseline concentrations for nine months.

RESULTS:

Patients had received prior ZDV therapy for an average of 15.5 months before starting ddI. Mean CD4+ cell counts increased from 53.9/mm3 at baseline to 72.4/mm3 after 4 months of therapy (p=0.04) but returned to concentrations comparable with those at baseline after 5 months. One case of documented pancreatitis, two cases of suspected pancreatitis, and nine cases of peripheral neuropathy occurred during ddI therapy. The estimated mean cumulative dose for the development of neuropathy was 1.16 g/kg, which is lower than previously reported.

CONCLUSIONS:

Patients who have received prolonged therapy with ZDV or who have low initial CD4+ counts may not have sustained increases in CD4+ counts from ddI therapy. Also, development of peripheral neuropathy may occur at lower cumulative doses in these patient populations.

Didanosine

OBJECTIVE:

To review the chemistry, intracellular metabolism, pharmacokinetics, and clinical experience with didanosine (2',3'-dideoxyinosine [ddI]).

DATA SOURCES:

English-language articles and conference proceedings (indexing terms were didanosine, 2′,3′-dideoxyinosine, and ddI).

STUDY SELECTION:

Available Phase I studies and abstracts determined to have clinical significance were included.

DATA EXTRACTION:

Clinical experience with ddI is limited to uncontrolled Phase I studies and a large “expanded-access” program. The primary outcome parameters used to evaluate ddI were the HIV surrogate markers: CD4+ lymphocytes and p24 antigen. Thus, the clinical data reviewed here must be evaluated critically and be considered preliminary until the results of studies comparing ddI with zidovudine (ZDV) and combination studies are available.

DATA SYNTHESIS:

Didanosine has been approved for the treatment of HIV infection in patients who are unable to tolerate ZDV because of adverse effects (e.g., anemia and neutropenia) or who experience clinical or immunologic deterioration while receiving ZDV. Compared with ZDV, ddI has a long intracellular half-life and negligible bone-marrow toxicity. It also has in vitro activity against ZDV-resistant strains of HIV. Phase I studies indicate that ddI has a beneficial effect on the CD4+ cell counts and HIV p24 antigen concentrations. As a result of the acid-labile nature of ddI, oral formulations are buffered or must be mixed with antacid to neutralize gastric pH. Bioavailability then averages 20–40 percent, depending on the dose and formulation given. The plasma half-life, total body clearance, and volume of distribution of ddI are one to two hours, 0.7–1 L/kg/h, and 0.8–1 L/kg, respectively. Painful peripheral neuropathy and pancreatitis (dose-limiting toxicities of ddI) occurred in 34 and 9 percent of patients in Phase I studies, respectively.

CONCLUSIONS:

Didanosine has demonstrated preliminary efficacy in the treatment of late-stage HIV infection; however, its effect on patient survival, its efficacy relative to ZDV, and its utility in combination with other agents are still under evaluation.

Didanosine

The evaluation and development of didanosine as a therapy for HIV-infection has demonstrated that results of cell-culture studies are only approximate predictors of clinical effects. Whereas the antiviral activity of didanosine is achieved in cell culture at concentrations much higher than those of zidovudine, the two drugs appear to achieve similar effects on markers of HIV-infection at similar clinical doses. The effect of the differing intracellular-half-life of the respective nucleoside triphosphates on the clinical effects is unknown. Similarly, preclinical toxicology studies, while suggesting a low potential for didanosine-induced haematological toxicities, did not predict the findings of peripheral neuropathy and pancreatitis in clinical studies. Thus, the results of controlled clinical studies are required in order to more fully define the therapeutic and safety profile of didanosine.

Treatment of Acute SIV infection in cynomolgus Monkeys with 2′,3′-dideoxyinosine (ddl) and 2′,3′-dideoxythymidiene (d4T)

2′,3′-dideoxyinosine (ddl) and 2′,3′-dideoxythymidiene (d4T) have been reported to inhibit the multiplication of human immunodeficiency virus (HIV) in patients, ddl and d4T were administered subcutaneously to cynomolgus monkeys starting 8 h prior to inoculation with 10–50 infectious doses of simian immunodeficiency virus (SIVSM). ddl, at a daily dose of 3 × 3.6 mg kg−1 for 10 days significantly delayed the appearance of SIVSM p24/26 antigen. A significant delay in the appearance of SIVSM p24/26 antigen was also seen when using a daily dose of 3 × 1 mg kg −1 of d4T for 10 days. Neither ddl nor d4T prevented SIVSM infection despite administration prior to virus inoculation.

Cutaneous toxicities of antiretroviral therapy for HIV: part I. Lipodystrophy syndrome, nucleoside reverse transcriptase inhibitors, and protease inhibitors

Antiretroviral medications for the treatment of HIV are common drugs with diverse and frequent skin manifestations. Multiple new cutaneous effects have been recognized in the past decade. Dermatologists play an important role in accurately diagnosing and managing the cutaneous toxicities of these medications, thereby ensuring that a patient has as many therapeutic options as possible for life-long viral suppression. Part I of this two-part series on the cutaneous adverse effects of antiretroviral medications will discuss HIV-associated lipodystrophy syndrome, which can be seen as a result of many antiretroviral medications for HIV, and the specific cutaneous effects of the nucleoside reverse transcriptase inhibitors and protease inhibitors.

Mannan-coated gelatin nanoparticles for sustained and targeted delivery of didanosine: in vitro and in vivo evaluation

Macrophages of the reticuloendothelial system and brain act as major reservoir for HIV because of their long term survival after HIV infection and ability to spread virus particles to bystander CD4 positive lymphocyte cells. The objective of the present study was to investigate mannan-coated nanoparticles for macrophage targeting of didanosine. Different didanosine loaded nanoparticles were prepared using the double desolvation technique and were characterized in vitro, ex vivo and in vivo. Results of the ex vivo cellular uptake study indicated 5-fold higher uptake of didanosine from the mannan-coated nanoparticles formulation (62.5 +/- 5.4%) by the macrophages in comparison with didanosine solution in phosphate buffer saline (PBS, pH 7.4) (12.1 +/- 2.3%). The better cellular uptake from the nanoparticles formulation was further confirmed by fluorescence microscopy using hydrophilic 6-carboxyfluorescein as a marker. Results of the quantitative biodistribution study showed 1.7, 12.6 and 12.4 times higher localization of didanosine in the spleen, lymph nodes and brain, respectively, after administration of mannan-coated nanoparticles compared to that after injection of didanosine solution in PBS (pH 7.4). Results of the present study showed that the mannan-coated nanoparticles targeted didanosine to the macrophage by mannosyl receptor mediated endocytosis.

Oral sustained-release bioadhesive tablet formulation of didanosine

The objective of this study was to formulate a hydrogel-forming bioadhesive drug delivery system for oral administration of didanosine (ddI). The aim of this tablet dosage form is to improve the oral absorption of ddI by delivering it in small doses over an extended period and localizing it in the intestine by bioadhesion. Compressed tablets of ddI using Polyox WSRN-303, Carbopol 974P-NF, and Methocel K4M as the bioadhesive release rate-controlling polymers were prepared. The effect of polymer concentration on the release profile and in vitro bioadhesion of the matrix tablets was studied. Tablet formulations with Polyox WSRN-303 (10%) and Methocel K4M (30%) showed 93 and 90% drug release, respectively, after 12 h. The drug release was found to be linear when fitted in the Higuchi equation (square-root time equation), suggesting zero-order release. Carbopol 974-P-NF was found to inhibit the complete release of ddI because of drug-polymer interaction; hence, is not suitable for formulation of ddI. Drug diffusion and swelling of the polymer (anomalous Fickian release) was found dominant in ddI release. In general, in vitro bioadhesion increased with an increase in polymer concentration. Tablets containing a single polymer can be designed to form hydrogels serving the dual purpose of bioadhesion and sustained release.

Didanosine once daily: potential for expanded use

Factors affecting patient adherence to therapy, such as frequent daily dosing and complex dosing schedules, are widely understood to be key obstacles to the durability of effective anti-HIV therapy. Didanosine, a nucleoside analogue reverse transcriptase inhibitor (NRTI) that is a core component of combination antiretroviral regimens, is currently indicated for twice-daily dosing. However, the active metabolite of didanosine (2',3'-dideoxyadenosine-5'-triphosphate) has a long intracellular half-life that supports the use of didanosine in a more patient-friendly, once-daily dosing schedule. Clinical studies in which didanosine was administered either once or twice daily, as monotherapy or in combination with another NRTI, have demonstrated the equivalence of both dosing schedules, with respect to safety and tolerability, virologic and immunologic endpoints, and short-term clinical effects (e.g., weight gain). Preliminary results from recent studies support the clinical efficacy and utility of once-daily didanosine in combination antiretroviral regimens that provide maximal drug exposure, while allowing for once- or twice-daily dosing of all component drugs.

Temporal development of 2',3'-dideoxyinosine (ddI)-induced peripheral myelinopathy

The anti-HIV therapeutic dideoxyinosine (ddI) has been reported to produce a painful, dose-limiting peripheral myelinopathy in HIV-infected patients after chronic administration. We have previously demonstrated ddI-induced myelinopathy in a non-HIV-infected rat model after 20 weeks of dosing, characterized by myelin splitting and intramyelin edema. The present study examined the time course needed to produce the ddI-induced neuropathy. Adult male Sprague-Dawley rats were gavaged with vehicle or 415 mg/kg ddI twice daily for up to 20 weeks. Groups of treated (n = 6-8) and control (n = 3-5) animals were killed after 5, 10, 15, and 20 weeks of dosing and the distal end of the sciatic nerve was removed. The nerve was postfixed by immersion in neutral phosphate-buffered formalin, dehydrated in graded alcohols, and embedded in plastic embedding media. One-micrometer-thick sections were cut and stained with toluidine blue and basic fuchsin. Plasma levels of ddI on the day the animals were killed were greater than 10 microgram/ml within the first hour after dosing and fell rapidly to less than 1 microgram/ml (clinical range 1-2 microgram/ml) within 3 h after dosing. The abnormalities observed in the sciatic nerve were few, if any, after 5 or 10 weeks, but very prominent after 15 weeks of dosing. Four of the six ddI-treated rats exhibited abnormal morphology as evidenced by myelin splitting and ballooned myelin sheaths. Although abnormal morphology was present at 20 weeks of dosing, the effect was not as robust as at 15 weeks. This suggests that the nerve may partially recover from the effects of ddI with time. Published by Elsevier Science Inc.

Virological and immunological responses to once-daily dosing of didanosine in combination with stavudine. AI454-143 Team

OBJECTIVE

To compare the antiviral activity of once-daily didanosine (ddI) and twice-daily ddI in combination with stavudine (d4T).

DESIGN

Randomized, double-blind, multicenter study.

SETTING

Twenty-one sites in the United States.

PATIENTS

Eighty-seven antiretroviral-naive, HIV-1-infected adults with baseline plasma HIV RNA counts of > or = 10,000 copies/ml and CD4 cell counts of > or = 100 cells/mm3 started study therapy.

INTERVENTIONS

Patients received once-daily ddI or twice-daily ddI, each combined with twice-daily d4T.

MAIN OUTCOME MEASURES

Plasma HIV-1 RNA levels, CD4 cell counts, and adverse events were regularly monitored. The primary efficacy analysis used was the time-averaged difference (TAD) between treatment regimens in change from baseline plasma HIV-1 RNA levels over the first 12 weeks of therapy.

RESULTS

At week 12, median log10 HIV-1 RNA changes were -1.83 log10 copies/ml in the once-daily ddI/d4T group and -1.80 log10 copies/ml in the twice-daily ddI/d4T group, and 18 out of 44 patients (41%) and 17 out of 43 patients (40%), respectively, had HIV-1 RNA levels below 400 copies/ml. Similar results were seen at week 24. The TAD between the two treatment groups (once-daily ddI/d4T minus twice-daily ddI/d4T) in change from baseline plasma HIV RNA levels over the first 12 weeks was 0.14 log10 copies/ml (95% CI: -0.11, 0.40). At week 12, subjects averaged an increase in CD4 cell count of over 140 cells/mm3. The TAD between the two treatment groups in change from baseline CD4 cell counts over the first 12 weeks was 2 cells/mm3 (95% CI: -40, 45).

CONCLUSION

Once-daily ddI plus d4T and twice-daily ddI plus d4T were similarly effective in reducing plasma HIV-1 RNA levels and increasing CD4 cell counts over 12-24 weeks of therapy.

Acute pancreatitis in human immunodeficiency virus-infected patients: a review

Acute pancreatitis is a clinical condition that develops when active pancreatic inflammation is induced by stimuli noxious to the pancreas. Patients infected with human immunodeficiency virus (HIV) often have histologic abnormalities of the pancreas, and acute pancreatitis is much more common in HIV-infected patients than in the general population. This article reviews the epidemiology and etiology of acute pancreatitis in HIV-infected patients. The clinical presentation and treatment of acute pancreatitis in HIV-infected patients are also reviewed.

Comparison of Once and Twice Daily Dosing of Didanosine in Combination with Stavudine for the Treatment of HIV-1 Infection

Objective

To compare the antiviral activity, safety and tolerability of didanosine dosed once and twice daily when administered in combination with stavudine dosed twice daily in human immunodeficiency virus type 1 (HIV-1)-infected individuals with little or no previous exposure to antiretroviral drugs.

Design

Comparative, multicentre, randomized, open-label, short-term study.

Patients and Methods

Eighty-four HIV-1-infected adults with qualifying baseline CD4 cell counts of 200 to 500 cells/mm3 were included in the study. Of these, 43 patients received once daily didanosine plus twice daily stavudine (group A) and 41 subjects received twice daily didanosine plus twice daily stavudine (group B). The primary efficacy analysis used was the time-averaged difference (TAD) between treatment regimens of variations in plasma HIV-1 RNA levels from baseline over the first 12 weeks of therapy. Plasma HIV-1 RNA levels, CD4 cell counts and adverse events were monitored. Results: At week 12, median HIV-1 RNA variations were –1.18 log10 copies/ml in group A and –0.88 log10 copies/ml in group B. For patients who were followed up to week 24, median variations of HIV-1 RNA levels from baseline were –1.21 log10 copies/ml in group A and –0.78 log10 copies/ml in group B. The TAD between the two treatment groups for variations from baseline plasma HIV-1 RNA levels over the first 12 weeks was 0.10 log10 copies/ml (95% confidence interval, –0.19 to 0.40), indicating equivalence.

Conclusion

Once daily didanosine plus twice daily stavudine and twice daily didanosine plus twice daily stavudine are equally effective in reducing plasma HIV-1 RNA levels and increasing CD4 cell counts. Both regimens are safe and well tolerated.

The pancreas in AIDS

Although autopsy studies reveal significant pancreatic lesions in about 10% of AIDS patients, pancreatic lesions infrequently produce symptoms and are rarely recognized premortem. Patients with AIDS can develop pancreatic disease from causes not related to AIDS or AIDS-specific lesions. AIDS-specific causes include opportunistic infection, AIDS-associated neoplasia, and medications used to treat complications of AIDS. Pancreatic involvement is usually part of a widely disseminated tumor and rarely produces clinical symptoms.

Switching from zidovudine to didanosine in patients with symptomatic HIV infection and disease progression. ddI Iberian Study Group

This study evaluated the efficacy of switching to didanosine in patients who were clinically or immunologically progressing despite zidovudine therapy. This multicenter, open-label study involved 400 patients with the acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC), who had tolerated zidovudine for at least 12 weeks and had signs of clinical or immunological disease progression. They were randomly assigned to receive 600 mg/d of zidovudine (n=133), 500 mg/d of didanosine (n=131), or 200 mg/d of didanosine (n=136). The primary end point was a new AIDS-defining event or death. The study was prematurely terminated, after the first interim analysis, mainly owing to results of two controlled studies demonstrating that a change to didanosine was associated with an improved outcome in patients with advanced HIV-1 disease. The median duration of follow-up was 53 weeks. The primary end point rates were 41, 58, and 59 (per 100 person-years) in the didanosine 500 mg, didanosine 200 mg, and zidovudine groups (zidovudine vs. didanosine 500 mg, relative risk 1.28, 95% confidence interval, 0.88-1.86, p = 0.19; didanosine 200 vs. 500 mg, relative risk 1.24, 95% confidence interval, 0.85-1.79, p = 0.26). In subjects with a baseline CD4 count of 100/mm3 or more, the primary end point rates were 8, 29, and 25 (per 100 person-years) in the didanosine 500 mg, didanosine 200 mg, and zidovudine groups, respectively (zidovudine vs. didanosine 500 mg, relative risk 2.96, 95% confidence interval 0.91-9.62, p = 0.07). No difference was seen in survival. In the didanosine 500 mg group, more patients had a 50% increase in CD4 cells (10% vs. 1% in zidovudine group, p = 0.01) and an increase of > or = 2.5 kg in body weight (2% versus 3%). Fatal pancreatitis developed in one patient assigned to didanosine 500 mg and in one to zidovudine. Our data suggest that switching from zidovudine to currently recommended doses of didanosine in subjects with ARC or AIDS who show evidence of clinical and laboratory disease progression can be associated with improvements in clinical outcome as well as in surrogate markers of HIV disease progression. This effect tended to be greater among individuals with higher CD4 counts (>100/mm3).

Drug-induced pancreatitis

Few data exist about the incidence of drug-induced pancreatitis in the general population. 20 cases of drug-related pancreatitis were reported in Switzerland over a period of 12 years. The proportion of cases of pancreatitis caused by drugs is estimated to be around 2% in the general population, with much higher proportions in specific subpopulations, such as children and patients who are HIV positive. The literature about drug-induced pancreatitis consists mainly of anecdotal case reports. Clear evidence of a definite association with pancreatitis, by means of rechallenge tests, or consistent case reports, supported by animal experiments or data on the incidence of acute pancreatitis in drug trials exists for didanosine, valproic acid (sodium valproate), aminosalicylates, estrogen, calcium, anticholinesterases and sodium stibogluconate. An association with drug-induced pancreatitis is likely but not definitely proven for thiazide diuretics, pentamidine, ACE inhibitors, asparaginase, vinca alkaloids, some nonsteroidal anti-inflammatory drugs and clozapine. Pancreatitis is possibly caused by azathioprine, furosemide (frusemide), tetracycline, metronidazole, isoniazid, rifampicin (rifampin), sulphonamides, cyclosporin and some antineoplastic drugs. Many drugs have been reported to be associated with acute pancreatitis. However, lack of rechallenge evidence, consistent statistical data, or evidence from experimental studies on a possible mechanism prohibit definitive conclusions about most of them. The high incidence of concurrent illnesses known to induce acute pancreatitis, makes a trigger role or co-factor role for the drug seem most likely.

Pancreatic disorders in pediatric acquired immune deficiency syndrome

Acute pancreatitis, reported in 17% of pediatric patients with acquired immune deficiency syndrome (AIDS), is said to have a poor prognosis. We describe the pancreatic changes observed at autopsy from 71 children with human immunodeficiency virus (HIV) infection and document their nature, extent, and clinical relevance. The median age at autopsy of the children was 17 months (range, 2 months to 19 years); 38 were boys and 33 were girls. Parental intravenous drug use was the most frequent risk factor for AIDS, followed by blood transfusions. Respiratory failure and sepsis constituted the predominant causes of death. Nonspecific changes, such as edema, inflammation, fibrosis, inspissated material in acini and ducts, and enlarged Langerhans' islet predominated. Acute and chronic pancreatitis were mild except in one instance of a fatal acute probably dideoxyinosine-associated pancreatitis. Pancreatic involvement by opportunistic infections, such as cytomegalovirus (CMV), Mycobacterium avium intracellulare (MAI), and Candida, was focal and rare despite the high prevalence of these infections at autopsy. Focal lymphoplasmacytic infiltration and vascular calcifications were also observed. We conclude that pancreatic changes were frequently noted at autopsy in children with AIDS. They were usually mild, reflected systemic disease states, and were usually not life threatening. The incidence of opportunistic infections of the pancreas was low.

Pancreatitis during intravenous pentamidine therapy in an AIDS patient with prior exposure to didanosine

OBJECTIVE

To report a case of an HIV-positive man who received sequential didanosine and pentamidine treatment and subsequently developed acute clinical pancreatitis.

CASE SUMMARY

In June 1992 didanosine 200 mg po bid was initiated in a 30-year-old man with AIDS. After a 22-week course of didanosine, the patient was hospitalized and didanosine was discontinued on day 4. The patient then received 8 days of treatment for a presumed Pneumocystis carinii pneumonia (PCP) with pentamidine 4 mg/kg/d iv. As the patient responded clinically to therapy, he was discharged home to complete a 21-day course of pentamidine. On day 14 of therapy, the patient experienced nausea, vomiting, diarrhea, fatigue, and was hypotensive. The dosage of pentamidine was reduced by 50 percent. After receiving 18 doses of pentamidine, treatment was discontinued, as symptoms had worsened and serum amylase and lipase concentrations were elevated. The patient was hospitalized and the diagnosis of acute clinical pancreatitis was made. After a 21-day hospitalization, the patient was discharged home in fair condition on hyperalimentation.

DISCUSSION

Potential causes of pancreatitis, including opportunistic infections, neoplasms, and drugs, are discussed. The most probable factors associated with pancreatitis in our patient are didanosine and pentamidine therapy.

CONCLUSIONS

As our patient developed pancreatitis following sequential administration of didanosine and pentamidine, it would be prudent to monitor for signs and symptoms of pancreatitis in similar cases. In addition, didanosine should be discontinued during and for one week following treatment of PCP when pentamidine is used.

Fulminant hepatitis with severe lactate acidosis in HIV-infected patients on didanosine therapy

We report two cases of fulminant hepatic failure in HIV-1-infected patients treated with didanosine (ddI). Clinical manifestations including vomiting, diarrhoea and dyspnoea were identical in both cases. Biological data mainly revealed hepatic failure and lactic acidosis. Histological examination of liver biopsies showed diffuse microvesicular steatosis. The outcome was fatal in both patients. The only comparable case previously reported (Lai et al., 1991) showed close similarities in the clinical, biological and histological manifestations with microvesicular steatosis. This prompted us to suspect that ddI might be responsible for fulminant hepatitis in all three AIDS patients. This toxic effect may be added to the list of potential adverse events occurring during ddI therapy.

Prodrugs of 2',3'-didehydro-3'-deoxythymidine (D4T): synthesis, antiviral activity, and rapid pharmacokinetic evaluation

A series of 5'-derivatives and modified pyrimidine analogues of 2',3'-didehydro-3'-deoxythymidine (d4T, stavudine, 1) were synthesized to determine their potential as oral prodrugs of d4T. Utilizing a screen developed for the rapid evaluation of a variety of prodrugs in mice, it was determined that 5'-acetate 2 provided comparable plasma levels of d4T after oral administration of the prodrug to that when d4T was administered alone. The relative oral bioavailability of methoxy acetate 3 and cyclohexyl carbonate 5 was 79 and 41%, respectively. Dihydropyridine ester 6 did not provide detectable levels of d4T up to 1 h after oral administration of 6. Thiopyrimidines 8 and 9, as well as aminopyrimidine 10 also failed to provide measurable levels of d4T after oral administration. 5'-Derivatives 3, 5, and 6 showed similar activity to that of d4T against HIV and MuLV, as did 5'-benzoyl-4-thio derivative 8. However, the corresponding 4-thio 5'-alcohol 9 was inactive.

Didanosine-associated eosinophilia with acute thrombocytopenia

OBJECTIVE

To report a case of didanosine (ddI)-induced acute thrombocytopenia and to discuss the incidence of this occurrence.

CASE SUMMARY

A 42-year-old man was admitted to the hospital on November 21, 1991, with acute thrombocytopenia. The patient had an AIDS index diagnosis of Kaposi's sarcoma (KS) in 1988 and had been taking zidovudine (ZDV) for three years without any adverse effects. On July 26, 1991, he complained of increasing fatigue and weight loss. At that time, the therapy was changed from ZDV to ddI. Over the next four months the drug therapy was altered from ddI to ZDV and ZDV to ddI because of increasing fatigue and gastrointestinal symptomatology; all of the other chronic medications were maintained at the same dosages and frequency. All laboratory work remained stable and no new active processes were found during this period. The patient developed acute thrombocytopenia upon his last change to ddI and all of the chronic drug therapy was stopped. His platelet count returned to his baseline value three weeks after the discontinuation of ddI and reinstitution of his chronic medications.

DISCUSSION

A review of the literature showed two documented cases of thrombocytopenia with ddI on a dosage escalation study.

CONCLUSIONS

Close patient monitoring is essential while administering ddI.

Didanosine, a new antiretroviral drug. A review

In this article the literature about didanosine, an antiretroviral drug, is reviewed. The mechanism of action, biochemical pharmacology, pharmacokinetics, and clinical results of phase-I trials are discussed. Serious adverse effects such as pancreatitis and peripheral neuropathy have occurred in these trials. An antiretroviral effect was observed in terms of an increase in CD4+ lymphocytes and a decrease in p24 antigen levels in HIV-infected individuals. Didanosine seems to be a promising drug against HIV infection, but knowledge about its clinical efficacy is scanty.

Impact of bioavailability on determination of the maximal tolerated dose of 2',3'-dideoxyinosine in phase I trials

The objective of this study was to determine the population pharmacokinetic parameters and the extent of absorption of 2',3'-dideoxyinosine, a nucleoside analog with activity against human immunodeficiency virus in vitro and in vivo, after oral and intravenous administration through the use of NON-linear Mixed Effects Modeling. The data were drawn from the pharmacokinetics section of an open-label, multicenter phase I study. One center administered ddI on a once-daily schedule. The other centers administered the drug once every 12 h. Drug was administered intravenously, and the plasma concentration-time profile was determined. Patients were then given the drug orally at twice the dose used in the intravenous portion of the study, and the pharmacokinetic profile was again determined. A 40-fold range of doses was examined. Forty-six human immunodeficiency virus-infected patients were studied. Concentrations in plasma were determined by high-pressure liquid chromatography. Clearance of the drug from plasma was 47.7 liters/h/70 kg of body weight. The terminal half-life was 1.4 h. The volume of distribution in the central compartment was 18.8 liters/70 kg. Absorption was rapid, with an absorption half-life of 0.52 h. Bioavailability with once-daily administration was 27%. For twice-daily administration, bioavailability rose to 36%. This difference was significant (P much less than 0.01). For doses of less than or equal to 5.1 mg/kg given every 12 h (10.2 mg/kg/day), bioavailability was 41%. We conclude that once-daily administration results in lower mean bioavailability, probably because of a saturation of the absorption process similar to that seen with acyclovir. This difference in bioavailability on the basis of the administration schedule explains the different short-term maximal tolerated doses identified in phase I trials of this agent.

Viruses, chemotherapy and immunity

An increasing number of antiviral agents are presently in various stages of development and testing, and an increasing number have recently been licensed for use in humans. These drugs have been used extensively to treat viral infections in immunocompromised individuals, and these studies indicate that for many antiviral agents the response to therapy is highly dependent on the integrity of the underlying host immune response. In particular, the response to zidovudine, acyclovir and ganciclovir in persons with HIV-1 infection is highly dependent upon CD4 number, which can be considered a surrogate marker for the state of host immune function in these subjects. Responses to interferons likewise can be shown to depend on the host immune response, with responses due to both direct antiviral effects of this agent as well as immunomodulatory effects mediated through interferon-induced upregulation of HLA molecule expression. The interdependence of host immunity with antiviral efficacy is underscored by the increased antiviral drug resistance in persons with advanced degrees of chronic immunosuppression, related to the higher level of viral replication and viraemia which occurs in the absence of an effective host immune response. Further definition of the precise mechanisms of these interactions should facilitate the rational design of antiviral agents and immunomodulatory therapies to improve treatment of viral infections.

Pharmacokinetics of 2',3'-dideoxyinosine in monkeys

Comparison of the pharmacokinetic parameters obtained for dideoxyinosine (ddI) in monkeys with those obtained for humans indicates that the monkey is an appropriate animal model for ddI pharmacokinetics in humans. Following intravenous administration of 20 mg of ddI per kg of body weight and measurement of ddI in plasma and urine by high-performance liquid chromatography, pharmacokinetic parameters were determined by noncompartmental analysis. Total systemic clearance was 0.74 liters/h/kg, volume of distribution was 0.92 liters/kg, and the elimination half-life of ddI was 1.22 h. The pharmacokinetics of ddI in five monkeys were determined following intravenous administration of 20 mg/kg and were found to be comparable to those obtained in patients with AIDS.