Effects of spontaneous gastric hypoacidity on the pharmacokinetics of zidovudine and didanosine

STUDY OBJECTIVE

To determine the effect of spontaneous gastric hypoacidity on the pharmacokinetics of zidovudine and didanosine in subjects infected with the human immunodeficiency virus (HIV).

DESIGN

Controlled, open-label, single-dose, pharmacokinetic study.

SUBJECTS

Thirty-two asymptomatic HIV-infected subjects.

INTERVENTIONS

Gastric pH studies were conducted in all 32 subjects, and 20 of these subjects (8 women, 12 men) were enrolled into the pharmacokinetic study. They were stratified into two groups according to fasting gastric pH: those without and with gastric hypoacidity (minimum gastric pH < 3 and > or = 3, respectively). Gastric pH was measured using the Heidelberg pH monitoring system in all subjects before and during pharmacokinetic analysis of zidovudine 100 mg or didanosine 200 mg (given as two 100-mg tablets dissolved in 6 oz water). Plasma samples were collected over 8 hours after dosing.

MEASUREMENTS AND MAIN RESULTS

Six (20%) of 30 subjects had a minimum gastric pH of 3 or above on at least two occasions, and the remaining 2 had variable gastric pH. Although gastric pH was unchanged during the administration of zidovudine, it increased to greater than 9 in 11 of 12 subjects with didanosine, regardless of baseline value. For both drugs, there were no statistically significant differences in peak plasma concentration (Cmax), time to reach peak plasma concentration (Tmax), elimination rate constant (ke), and area under the plasma concentration-time curve from time zero to infinity (AUC0-infinity) between subjects with and without gastric hypoacidity despite sufficient statistical power to detect a 56% difference in clearance for either drug (alpha 0.05, beta 0.1).

CONCLUSION

Gastric hypoacidity occurs in approximately 20% of HIV-infected patients and does not appear to influence zidovudine or didanosine pharmacokinetics.

In vivo evidence for carrier-mediated efflux transport of 3'-azido-3'-deoxythymidine and 2',3'-dideoxyinosine across the blood-brain barrier via a probenecid-sensitive transport system

By analyzing the amount of ligand remaining in the brain after microinjection into the brain cortex, the apparent efflux rate constants (Keff) of 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxyinosine (DDI) across the blood-brain barrier at low concentrations were determined to be 0.0317 +/- 0.0068 min(-1) and 0.0253 +/- 0.0037 min(-1), respectively. At higher concentrations, efflux exhibited saturation. The concentration of unlabeled DDI to inhibit 50% of the saturable efflux of [3H]DDI was found to be 11.3 +/- 5.7 microM, assuming that DDI diffused into the same volume of brain as that of trypan blue after intracerebral administration. The efflux rate of [3H]AZT from the brain was significantly inhibited by DDI, probenecid, p-aminohippuric acid, benzylpenicillin and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, but not by thymidine. Moreover, the efflux rate of [3H]DDI was significantly inhibited by AZT and probenecid, but not by deoxyinosine and inosine. After intracerebroventricular injection, the apparent efflux clearances of [3H]AZT and [3H]DDI from the cerebrospinal fluid were significantly inhibited by the coadministration of probenecid. However, intracerebroventricularly administered probenecid had no effect on the efflux of [3H]AZT and [3H]DDI from the brain after intracerebral microinjection, which suggested that the efflux transport system of the blood-cerebrospinal fluid barrier is not responsible for the elimination of AZT and DDI from the cerebral cortex. These results provide kinetic evidence that AZT and DDI are transported from brain into circulating blood across the blood-brain barrier via a probenecid-sensitive carrier-mediated efflux transport system.

Physiologically based pharmacokinetic models of 2',3'-dideoxyinosine

PURPOSE

The goal of this study was to develop physiologically based pharmacokinetic (PBPK) models for 2',3'-dideoxyinosine (ddI) in rats when the drug was administered alone (ddI model) and with pentamidine (ddI + pentamidine model), and to use these models to evaluate the effect of our previously reported pentamidine-ddI interaction on tissue ddI exposure in humans.

METHODS

The PBPK models consisted of pharmacologically relevant tissues (blood, brain, gut, spleen, pancreas, liver, kidney, lymph nodes, muscle) and used the assumptions of perfusion-rate limited tissue distribution and linear tissue binding of ddI. The required physiologic model parameters were obtained from the literature, whereas the pharmacokinetic parameters and the tissue-to-plasma partition coefficients were calculated using plasma and tissue data.

RESULTS

The ddI model in rats yielded model-predicted concentration-time profiles that were in close agreement with the experimentally determined profiles after an intravenous ddI dose (5% deviation in plasma and 20% deviation in tissues). The ddI + pentamidine model incorporated the pentamidine-induced increases of ddI partition in pancreas and muscle. The two PBPK models were scaled-up to humans using human physiologic and pharmacokinetic parameters. A comparison of the model-predicted plasma concentration-time profiles with the observed profiles in AIDS patients who often received ddI with pentamidine showed that the ddI model underestimated the terminal half-life (t1/2, beta) by 39% whereas the ddI + pentamidine model yielded identical t1/2, beta and area-under-the-curve as the observed values (< 1% deviation). Simulations of ddI concentration-time profiles in human tissues using the two models showed that pancreas and lymph nodes received about 2- to 30-fold higher ddI concentration than spleen and brain, and that coadministration of pentamidine increased the AUC of ddI in the pancreas by 20%.

CONCLUSIONS

Data of the present study indicate that the plasma ddI concentration-time profile in patients were better described by the ddI + pentamidine model than by the ddI model, suggesting that the pentamidine-induced changes in tissue distribution of ddI observed in rats may also occur in humans.

Phase I dose-escalation pharmacokinetics of AZT-P-ddI (IVX-E-59) in patients with human immunodeficiency virus

3'-Azido-3'-deoxythymidilyl-(5',5')-2',3'-dideoxy-5'-inosinic acid (AZT-P-ddI, IVX-E-59, Scriptene) is a heterodimer composed of one molecule of 3'-azido-3'-deoxythymidine (zidovudine or AZT) and one molecule of 2',3'-dideoxyinosine (didanosine or ddI) linked through their 5' positions by a phosphate bond. AZT-P-ddI exhibits enhanced antiviral activity and selectivity in vitro compared with AZT and ddI alone. The pharmacokinetics of AZT-P-ddI were studied in 12 patients with human immunodeficiency virus (HIV) who had CD4+ cell counts higher than 200 cells/mm3. Isotopic preparations of (14C)-AZT-P-(3H)-ddI were administered intravenously (50 mg and 100 mg) to eight patients; 1 month later these patients were crossed over to oral administration (100 mg and 200 mg). A second group of patients (n = 4) received only a 450-mg oral dose of AZT-P-ddI. Plasma levels of unchanged AZT-P-ddI after intravenous infusion declined rapidly and were undetectable 0.75 hours after the end of infusion, whereas the parent compound was not detected after oral administration, indicative of a very rapid metabolism. The parent entity was enzymatically cleaved in vivo yielding the two constituent drugs AZT and ddI, which were subsequently subjected to their respective pharmacokinetic and metabolic processes. The beta-glucuronide derivative of AZT (GAZT) represented the major metabolite of AZT, but there were no detectable levels of the toxic metabolite 3'-amino-3'-deoxythymidine (AMT). A major and previously unrecognized in vivo metabolite of ddI, referred as ddI-M, was detected in plasma and urine. Analysis by high-field proton nuclear magnetic resonance and mass spectrometry led to the identification of ddI-M as being R(-)-dihydro-5-(hydroxymethyl)-2(3H)-furanone. The formation of AZT and ddI metabolites was increased after oral administration of AZT-P-ddI compared with the intravenous infusion, with an area under the concentration-time curve (AUC) ratio of metabolite to AZT and metabolite to ddI being 7.7 and 5.7 (oral) and 3.8 and 1.1 (intravenous), respectively. The newly identified ddI-M exhibited sustained plasma levels for extended time periods with an apparent elimination half-life (t1/2) of approximately 10 hours after oral administration of AZT-P-ddI. Recovery of radioactivity associated with 3H and 14C in urine was essentially complete within 48 hours after oral and intravenous administration of AZT-P-ddI. The oral bioavailability of AZT (64.7-67.3%) and ddI (33.6-42.9%) and the other pharmacokinetic parameters were consistent with previous data reported with each nucleoside analog alone or in combination therapy.

Single-dose pharmacokinetics of delavirdine mesylate and didanosine in patients with human immunodeficiency virus infection

Delavirdine is a nonnucleoside reverse transcriptase inhibitor with in vitro activity against human immunodeficiency virus type 1 (HIV-1) that is currently being evaluated in combination regimens with various nucleoside analogs, including didanosine. Due to the pH-dependent solubility of delavirdine, the buffering agents in didanosine formulations may reduce delavirdine absorption. To evaluate the potential interaction between these agents, 12 HIV-infected patients (mean [+/- standard deviation] CD4+ cell count, 304 +/- 213/mm3) were enrolled in a three-way crossover single-dose study. Didanosine (125 to 200 mg given as buffered tablets) and delavirdine mesylate (400 mg) pharmacokinetics were evaluated when each drug was given alone (treatments A and B, respectively), when the two drugs were given concurrently (treatment C), and when didanosine was given 1 h after delavirdine (treatment D). Delavirdine exposure was reduced by concurrent administration of didanosine. The maximum drug concentration in serum (Cmax) was reduced from 7.22 +/- 4.0 to 3.51 +/- 1.9 microM, and the area under the concentration-time curve from 0 h to infinity (AUC0-->infinity) was reduced from 22.5 +/- 14 to 14 +/- 5.7 microM.h. The extent of N-dealkylation, as indicated by the ratio of the N-dealkylated delavirdine AUC0-->infinity to the delavirdine AUC0-->infinity, was unchanged across study treatments (P = 0.708). Reductions in didanosine exposure were observed during concurrent administration with delavirdine with a Cmax reduction from 4.65 +/- 2.0 to 3.22 +/- 0.59 microM and an AUC0-->infinity reduction from 7.93 +/- 3.9 to 6.54 +/- 2.3 microM.h. Thus, concurrent administration of delavirdine and didanosine may reduce the AUC0-->infinity of both drugs, although the clinical significance of this reduction is unknown. Administration of delavirdine 1 h before didanosine avoided the interaction. Due to the single-dose nature of this study, these findings require further evaluation at steady state.

A multiple-dose pharmacokinetic interaction study between didanosine (Videx) and ciprofloxacin (Cipro) in male subjects seropositive for HIV but asymptomatic

The pharmacokinetics of didanosine and ciprofloxacin were evaluated following the administration of multiple oral doses of each drug as a single agent or in combination. Didanosine was dosed as the Videx chewable/dispersible tablet, which contains the antacids dihydroxyaluminum sodium carbonate and magnesium hydroxide. Sixteen HIV-seropositive male subjects were randomly assigned to two groups of eight each. Group A received didanosine (200 mg q 12h) for 3d, followed by didanosine (200 mg q 12h) and ciprofloxacin (750 mg q 12h) for 3d, and finished with another course of didanosine (200 mg q 12h for 3 d). Group B began with ciprofloxacin, followed by the combination, and finished with ciprofloxacin using the same doses and schedule as utilized in group A. During the combination phase of the study, ciprofloxacin was administered 2 h prior to didanosine. Serial blood and urine samples were collected on study days 4, 8, and 12 for the quantitative determination of didanosine and ciprofloxacin using validated HPLC methods. The plasma and urine data were subjected to noncompartmental pharmacokinetic analysis. A statistically significant decrease in the average AUC and UR values of ciprofloxacin was noted when it was given with didanosine, relative to administration as a single agent. However, the magnitude of the decrease in these parameters, approximately 26 and 29%, respectively, was not considered clinically significant. The apparent decrease in the bioavailability of ciprofloxacin was probably due to the formation of a chelation complex between it and the aluminum- and magnesium-containing antacids found in the didanosine tablet. Other than an approximately 16% decrease in AUC, ciprofloxacin did not alter the pharmacokinetics of didanosine. The data from the present study demonstrate that didanosine or ciprofloxacin can be added to a treatment regimen consisting of the other single agent and that cessation of treatment with one agent does not have an impact on the pharmacokinetics of the other drug. The dose of ciprofloxacin must be taken at least 2h prior to didanosine to avoid a clinically significant interaction with the antacids present in the didanosine formulation.

Didanosine. An update on its antiviral activity, pharmacokinetic properties and therapeutic efficacy in the management of HIV disease

Didanosine is a dideoxynucleoside analogue, which is phosphorylated to the active metabolite dideoxyadenosine triphosphate (ddATP) intracellularly. At therapeutic concentrations, ddATP inhibits HIV replication by inhibiting HIV reverse transcriptase. Didanosine is established as a first-line treatment for patients with HIV disease and has recently been shown to be superior to zidovudine monotherapy in the treatment of patients with intermediate-stage HIV infection. In clinical practice, however, combination regimens of antiretroviral drugs are generally considered preferable to monotherapy as first-line treatment for most patients with HIV disease. Importantly, 2 large multicentre studies have demonstrated that combination therapy with didanosine and zidovudine was more effective than zidovudine monotherapy in delaying disease progression and death in patients with intermediate or advanced HIV disease. In other comparative studies, improvements in surrogate markers of HIV disease were generally greater in patients who received combination therapy than in recipients of antiretroviral drug monotherapy. Improvements in surrogate markers were also observed in children who received didanosine monotherapy in several clinical trials. Although the efficacy of combination antiretroviral drug therapy has not yet been investigated extensively in children, a combination regimen of didanosine and zidovudine was well tolerated and achieved beneficial effects on surrogate markers if HIV disease. In addition, preliminary findings of a larger study have shown that disease progression was delayed in children and adolescents who received didanosine plus zidovudine combination therapy compared with those receiving zidovudine monotherapy. Didanosine has a tolerability profile that is distinctly different from that of zidovudine. In particular, didanosine exhibits only minimal haematological toxicity when administered either as a single agent or in combination with zidovudine. The most serious dose-limiting adverse effects associated with didanosine treatment are peripheral neuropathy and pancreatitis. In conclusion, didanosine monotherapy is an effective treatment of HIV infection. However, combination antiretroviral therapy is the optimal treatment strategy for most patients, and didanosine is now firmly established as a component of combination antiretroviral drug regimens for the first-line treatment of patients with HIV disease.

A phase-I study of the safety, pharmacokinetics, and antiviral activity of combination didanosine and ribavirin in patients with HIV-1 disease. AIDS Clinical Trials Group 231 Protocol Team

A phase-I study was conducted to examine the safety, pharmacokinetics, and activity of combination 2',3'-dideoxyinosine (ddI) and ribavirin against human immunodeficiency virus type 1 (HIV-1)-positive individuals with CD4+ cell counts of < or = 500/microliter. Nineteen patients were enrolled into the study in which ddI monotherapy (200 mg p.o.b.i.d.) was administered for the first 4 weeks, followed by the coadministration of ribavirin (600 mg p.o.q.d.) and ddI (200 mg p.o.b.i.d.) for 8 or 20 additional weeks. The combination regimen was safe and well tolerated. Three patients did not complete 12 weeks of the study because of adverse events or voluntary withdrawal. The pharmacokinetic studies performed at weeks 4, 6, and 12 on specimens collected from the 15 individuals who completed 12 weeks of therapy revealed no pharmacokinetic interaction between ddI and ribavirin. A significant decline from baseline in HIV-1 titer as measured by quantitative HIV-1 culture was detected both during the ddI-monotherapy phase (week 4, p < 0.001) and during the combination-therapy ddI + ribavirin phase (week 12, p < 0.001); the median drop observed was 0.90 log10 at week 4 and 0.92 log10 at week 12. While the addition of ribavirin did not result in further reductions in viremia in the following weeks on study treatment, 13 (81%) of the 16 patients had at least a -0.5 log10 change in viral titer at week 12. The median decline in plasma viral RNA was 0.68 log10 at week 4(p < 0.001) and 0.67 log10 at week 12 (p = 0.005). CD4+ cell counts increased above baseline significantly during the ddI-monotherapy phase of the study (p = 0.0038). The median increase was +26 cells/mm3 at week 4 and +11 cells/mm3 at week 12; for patients who remained on treatment through 24 weeks, the median CD4+ cell count increase was +10 cells/mm3. The L74V ddI resistance-conferring HIV-I reverse-transcriptase mutation emerged in 53% of the patients. Patients with non-syncytium-inducing HIV variants demonstrated greater responses to treatment with larger decreases in virus load and greater increases in CD4+ cell count. Our results reveal that the combination of ddI and ribavirin in HIV-positive patients is safe, well tolerated, without adverse pharmacologic interaction, and associated with significant and sustained declines in virus load over 12 weeks of therapy.

Disposition of didanosine in HIV-seropositive patients with normal renal function or chronic renal failure: influence of hemodialysis and continuous ambulatory peritoneal dialysis

OBJECTIVE

To evaluate the pharmacokinetics of didanosine in patients with normal kidney function or chronic kidney failure.

METHODS

Three groups of patients with human immunodeficiency virus (HIV) infection were studied: group I, six men with normal kidney function (creatinine clearance > 90 ml/min/1.73 m2); group II, six men with chronic renal failure maintained on continuous ambulatory peritoneal dialysis (CAPD); and group III, four men and two women with chronic renal failure receiving hemodialysis three times a week. A 300 mg dose of didanosine was administered orally and intravenously according to a two-period randomized crossover design. Patients in group III were studied between hemodialysis sessions during the crossover periods. In addition, patients in group III were studied in a third period after administration of a 300 mg oral dose of didanosine 4 hours before hemodialysis.

RESULTS

After intravenous administration in group I, the mean (+/-SD) total clearance (CLT) was 13.0 +/- 1.6 ml/min/kg and the elimination half-life (t 1/2) was 1.56 +/- 0.43 hour. In groups II and III, the CLT decreased significantly to 3.4 +/- 1.2 and 3.2 +/- 1.2 ml/min/kg, respectively, whereas the t1/2 increased to 3.60 +/- 0.82 hours and 3.11 +/- 0.88 hours, respectively. The absolute bioavailability of didanosine in groups I, II, and III was 42% +/- 12%, 52% +/- 6%, and 38% +/- 11%, respectively, and did not differ significantly. CAPD had little effect on the removal of didanosine, whereas approximately 30% of the drug present in the body at the start of dialysis was eliminated by an average 3-hour dialysis session.

CONCLUSION

The clearance of didanosine is impaired in patients with chronic renal failure. To compensate, the dose and schedule of administration should be adjusted. It is recommended that one-fourth of the total daily dose of didanosine be administered once a day in this patient population.

Role of altered metabolism in dideoxynucleoside pharmacokinetics. Studies of 2'-beta-fluoro-2',3'-dideoxyinosine and 2'-beta-fluoro-2',3'-dideoxyadenosine in rats

Enzymes of the purine salvage pathway play an important role in altering the in vivo pharmacokinetics of 2',3'-dideoxypurine nucleosides. This study examines the pharmacokinetics of enzyme-resistant 2'-beta-fluoro analogues of 2',3'-dideoxyinosine (ddI) and 2',3'-dideoxyadenosine (ddA). 2'-beta-Fluoro-2',3'-dideoxyinosine (F-ddI) is an acid-stable analogue of ddI that is highly resistant to purine nucleoside phosphorylase, the principal enzyme in ddI metabolism. 2'-beta-Fluoro-2',3'-dideoxyadenosine (F-ddA), an acid-stable and purine nucleoside phosphorylase-resistant analogue of ddA, is converted in vivo to F-ddI by adenosine deaminase (ADA) but is a much poorer substrate for this enzyme than is ddA. Both F-ddA and F-ddI have been shown to have activity against human immunodeficiency virus in vitro, and F-ddA has been selected by the National Cancer Institute for clinical trials as a new human immunodeficiency virus reverse transcriptase inhibitor. The pharmacokinetics of F-ddI and ddI were compared at equivalent doses in chronically catheterized rats. Because ddI and F-ddI are isosteres having nearly identical lipophilicity, this comparison is likely to reflect primarily metabolic differences. The clearance of F-ddI was substantially reduced, in comparison with that of ddI (27.3 ml/min/kg vs. 90.9 ml/min/kg), resulting in higher systemic concentrations at steady state and prolonged retention of F-ddI after termination of infusions, consistent with a significant metabolic component in the clearance of ddI. Concentrations of F-ddA and F-ddI during and after infusions of F-ddA were determined in both untreated and 2'-deoxycoformycin-pretreated rats. In untreated rats, F-ddA was rapidly eliminated from plasma, with a total clearance of 68.5 ml/kg/min. Metabolic clearance of F-ddA to F-ddI accounted for 58% of this value (bioconversion t1/2 = 9.8 +/- 1.9 min). Pretreatment with 2'-deoxycoformycin, an ADA inhibitor, reduced the clearance of F-ddA to 23.8 ml/min/kg, leading to 2.9 +/- 0.4-fold higher steady-state plasma concentrations of F-ddA, in agreement with a 2.5-fold enhancement predicted by a compartmental model assuming complete ADA inhibition.

Prenatal and postpartum pharmacokinetics of stavudine (2',3'-didehydro-3'-deoxythymidine) and didanosine (dideoxyinosine) in pigtailed macaques (Macaca nemestrina)

Stavudine (5 mg/kg of body weight; n = 7) or didanosine (3.2 mg/kg; n = 4) was administered as an intravenous bolus to pregnant pigtailed macaques (Macaca nemestrina) near term and 4 to 5 weeks postpartum. No significant differences were found between the prenatal and postpartum total plasma drug clearance, steady-state volume of distribution, terminal plasma drug half-life, mean body residence time, or recovery of unchanged drug in urine. These data indicate that pregnancy does not affect the pharmacokinetics of stavudine or didanosine in M. nemestrina.

Viral load, CD4 percentage, and delayed-type hypersensitivity in subjects receiving the HIV-1 immunogen and antiviral drug therapy

Two trials of subjects inoculated with the inactivated, gp120-depleted HIV-1 Immunogen are reported. In one study, in which 19 subjects received ZDV and 8 subjects received ddI, treatment with the HIV-1 Immunogen did not affect the pharmacokinetic parameters of the antiviral drugs. In another study, 65 subjects who were previously immunized with the HIV-1 Immunogen over a mean period of 4.0 years (range, 1.2-5.4 years) received inoculations at 0 and 6 months. At some point during this 48-week study, 72% of the subjects (47/65) were receiving antiviral drug therapy. The HIV-1 DNA load in CD4 cells and CD4 percentage were found to be stable over the 48-week period. Delayed-type hypersensitivity to HIV-1 antigens increased after two inoculations with the HIV-1 Immunogen. In these two trials, no serious treatment-related adverse events were documented in the subjects. The two studies presented herein are the first to suggest that an immune-based therapy such as the HIV-1 Immunogen can be combined safely with antiviral drugs, supporting further study to evaluate the clinical utility of this approach.

High-performance liquid chromatographic analysis of 2'-fluoro-2',3'-dideoxyadenosine and 2'-fluoro-2',3'-dideoxyinosine in dog plasma and urine

A high-performance liquid chromatographic assay was developed and validated for a simulataneous determination of 2'-fluoro-2' 3'-dideoxyadenosine (FddA) and its metabolite, 2'-fluoro-2',3'-dideoxyinosine (Fddl) in dog plasma and urine. In vitro, FddA and Fddl exhibit activity against human immunodeficiency virus (HIV). A solid phase extraction was applied to extract FddA, Fddl, and the internal standard (IS; 3',5'-anhydrothymidine) from the biomatrices. The processed samples were chromatographed using a C8 column coupled with a mobile phase consisting of monobasic phosphate, dibasic phosphate, ethylene glycol monomethyl ether, and water. Detection was performed at 257 nm. The nominal retention times were 9, 14, and 26 min for Fddl, IS, and FddA, respectively. The lower limits of quantitation were 0.1 and 2.0 micrograms/mL in plasma and urine, respectively, for both analytes. The accuracy of the assay deviated < or = 10% from the nominal concentrations, and the precision was < or = 14% coefficient of variation. In either matrix, both analytes were stable for at least three freeze-thaw cycles and in the injection media for at least 54 h. The extraction recoveries of the analytes were greater than 80%. The application of this assay was demonstrated in a preliminary pharmacokinetic study of FddA and Fddl in dogs. Two male dogs per dose level received a 100, 250, or 500 mg/kg oral dose of FddA once daily for 14 days. The early appearance of Fddl in plasma (0.25 h; the first sampling time) and greater plasma levels of Fddl than FddA (> 50-fold of Cmax), suggested that the conversion of FddA to Fddl was rapid and extensive. Renal excretion appeared to be the major route of elimination of Fddl.

Combination therapy with stavudine and didanosine in children with advanced human immunodeficiency virus infection: pharmacokinetic properties, safety, and immunologic and virologic effects

OBJECTIVES

To obtain preliminary information on the pharmacokinetic properties, tolerance, safety, and antiviral activity of combination therapy with stavudine and didanosine in children with advanced human immunodeficiency virus (HIV) infection.

METHODS

Eight children (median age, 6.6 years; range, 2.8 to 12 years) with advanced HIV disease (median CD4 + lymphocyte count at baseline, 42 cells/ microL; range, 8 to 553 cells/microL) were treated with stavudine (2 mg/kg per day in two divided doses) and didanosine (180 mg/m2 per day in two divided doses) for 24 weeks. Seven children had histories of prior zidovudine therapy. All children had received stavudine alone for 19 to 33 months before the addition of didanosine to the treatment regimen. Children were assessed clinically and with laboratory studies at baseline, weekly through week 4 of combination therapy, and every 4 weeks thereafter.

RESULTS

Analysis of stavudine and didanosine plasma half-life values, clearances, and area under the plasma concentration-versus-time curves revealed no obvious clinical pharmacokinetic interaction between the drugs through study week 12. Combination therapy was well tolerated, and there were no drug-associated clinical or laboratory adverse events. Signs and symptoms of peripheral neuropathy were not observed. All three children with baseline CD4 + lymphocyte counts greater than 50 cells/muL had greater than 20% increases in their counts within the first 12 weeks of therapy; CD4 + lymphocyte count increases were not observed in the other children. Plasma HIV RNA concentrations showed median declines of 0.88 log10 (range, -3.41 log10 to 0.31 log10) and 0.30 log10 (range, -0.63 log10 to 0.89 log10) at study weeks 12 and 24, respectively.

CONCLUSIONS

Combination therapy with stavudine and didanosine was well tolerated and safe in this small group of children with advanced HIV disease. Plasma HIV RNA concentration declines suggest a favorable effect of therapy on virus load. These findings should be confirmed, and the regimen's clinical efficacy should be examined, in controlled studies of HIV-infected children with less-advanced disease.

Didanosine measurement by radioimmunoassay

Didanosine is commonly prescribed as monotherapy or as part of a combination regimen for patients with human immunodeficiency virus infection. The use of lower doses, either as part of a combination regimen or as a result of dose reduction secondary to clinical intolerance, requires that a sensitive assay method be available for either traditional or population-based pharmacokinetic evaluations. We evaluated a radioimmunoassay technique with a standard curve range of 0 to 100 ng/ml in human plasma, urine, and cerebrospinal fluid and assessed its accuracy and precision for use in pharmacokinetic studies.

Pharmacokinetic interaction between intravenous 2',3'-dideoxyinosine and pentamidine in rats

PURPOSE

This study examined the pharmacokinetic interaction between 2',3'-dideoxyinosine (ddI) and pentamidine.

BACKGROUND

ddI and pentamidine are often coadministered to patients with acquired immunodeficiency syndrome, and are both associated with pancreatic toxicity. Information on potential interaction would be useful to assess the need for dose modification and the basis of the higher incidence of pancreatic toxicity associated with coadministration of the two drugs.

METHODS

ddI (200 mg/kg) and pentamidine (10 mg/kg) were administered by continuous infusion to rats over 3 hr, either alone or concomitantly. Drug analysis was by high pressure liquid chromatography with UV or fluorescence detection, or by radioimmunoassay.

RESULTS

Pentamidine coadministration significantly increased the apparent volume of distribution at steady state of ddI from 1.4 to 3.4 l/kg (p = 0.004), and increased the mean residence time from 36.3 to 50.0 min (p = 0.015). Pentamidine enhanced the distribution of ddI from plasma into pancreas (p = 0.001) and muscle (p = 0.026). ddI distribution into spleen and liver was also increased, with differences approaching statistical significance (p = 0.08 and 0.06, respectively). In contrast, ddI coadministration did not affect the total body clearance but increased the urinary excretion and the renal clearance of pentamidine by about 5-fold (p = 0.0003).

CONCLUSIONS

These data indicate that pentamidine increased the distribution of ddI into pancreas and muscle, whereas ddI increased the renal elimination of pentamidine.

Intestinal absorption and first-pass elimination of 2', 3'-dideoxynucleosides following oral administration in rats

Intestinal absorption and first-pass elimination of 2',3'-dideoxynucleosides (ddNs), including 3'-azido-3'-deoxythymidine (AZT), 2',3'-dideoxyinosine (DDI) and 2',3'-didehydro-3'-deoxythymidine (D4T), following oral administration was investigated in rats. Enzymatic degradation of ddNs in rat intestinal washing and in the intestinal homogenate showed them to be stable in the washing with half lives of more than 140 h, whereas degradation of DDI in the intestinal homogenate was more than ten times as rapid as those of AZT and D4T. Intestinal absorption was studied in three segments of the rat intestine (duodenum, jejunum and colon) using an in situ closed-loop method. The area under plasma ddN concentration curve (AUC) and the residual percent of dose 1 h after dosing indicated a greater absorption of AZT and D4T in the upper intestinal tract than in the colon, very poor absorption of DDI in all segments, and considerable absorption of AZT in the colon. The AUC and the mean residence time (MRT) of ddNs following four different routes (intravenous: i.v., intra portal vein: i.p.v., intra duodenal: i.d. and intra gastric: i.g.) were measured using the in viva multiple sites of input method in rats. AZT and D4T were rapidly absorbed from the gastrointestinal tract and their bioavailability was more than 90%. DDI was less absorbed (33.02%) following i.d. administration compared with AZT and D4T. This poor absorption of DDI was partly attributable to its metabolism in the intestine.

A pharmacokinetic interaction study of didanosine coadministered with trimethoprim and/or sulphamethoxazole in HIV seropositive asymptomatic male patients

1. The pharmacokinetics of didanosine, trimethoprim, and sulphamethoxazole were evaluated in ten HIV seropositive asymptomatic patients as single agents and upon coadministration of single doses. 2. Using a randomized, balanced incomplete block crossover study with at least a 1-week washout period between successive treatments, each patient under fasting conditions received four of the following five treatments: 200 mg didanosine as a single agent; 200 mg trimethoprim + 1000 mg sulphamethoxazole; 200 mg trimethoprim + 200 mg didanosine; 1000 mg sulphamethoxazole + 200 mg of didanosine and; 200 mg trimethoprim + 1000 mg sulphamethoxazole + 200 mg didanosine. 3. Serial blood and urine samples were collected following the administration of each treatment. Plasma and urine samples were analyzed using high-pressure liquid chromatography (h.p.l.c.)/ultraviolet assays specific for unchanged didanosine, trimethoprim and/or sulphamethoxazole. 4. Percent urinary recovery (%UR) and renal clearance (CLR) emerged as consistently affected parameters, being decreased in the case of didanosine (35%, P = 0.016) and trimethoprim (32%, P = 0.019) and increased in the case of sulphamethoxazole (39%, P = 0.079), when all three agents were coadministered. The magnitude of the changes in didanosine CLR and %UR values was no greater when both trimethoprim and sulphamethoxazole were coadministered vs when each single agent was given with didanosine, suggesting that any effect was not additive. 5. Other key parameters such as Cmax, AUC, and t1/2 for didanosine (1309.9 ng ml-1, 1796.9 ng ml-1 h, and 1.61 h, respectively), trimethoprim (1.96 micrograms ml-1, 22.86 micrograms ml-1 h, and 9.03 h, respectively) or sulphamethoxazole (58.62 micrograms ml-1, 799.7 micrograms ml-1 h and 9.84 h, respectively) were not affected when didanosine was coadministered with either trimethoprim (didanosine: 1751.9 ng ml-1, 2158.0 ng ml-1 h, and 1.28 h; trimethoprim: 1.81 micrograms ml-1, 28.89 micrograms ml-1 h, and 11.4 h), sulphamethoxazole (didanosine: 1279.3 ng ml-1, 1793.2 ng ml-1 h, and 1.61 h; sulphamethoxazole: 53.57 micrograms ml-1, 732.1 micrograms ml-1 h, and 8.95 h), or the combination of trimethoprim and sulphamethoxazole (didanosine: 1283.7 ng ml-1, 1941.8 ng ml-1 h, and 1.38 h; trimethoprim: 1.59 micrograms ml-1, 26.68 micrograms ml-1 h, and 11.3 h; sulphamethoxazole: 59.48 micrograms ml-1, 760.9 micrograms ml-1 h, and 9.47 h). 6. Because the observed differences in CLR and %UR are small and not considered to be clinically relevant, it is not necessary to alter the dosing regimens of didanosine, trimethoprim or sulphamethoxazole when administered in combination to HIV seropositive patients.

Didanosine reduces atevirdine absorption in subjects with human immunodeficiency virus infections

Atevirdine is a nonnucleoside reverse transcriptase inhibitor with in vitro activity against human immunodeficiency virus type 1 and is currently in phase II clinical trials. Atevirdine is most soluble at a pH of < 2, and therefore, normal gastric acidity is most likely necessary for optimal bioavailability. Because of the rapid development of resistance in vitro, atevirdine is being evaluated in combination with didanosine and/or zidovudine in both two- and three-drug combination regimens. To examine the influence of concurrent didanosine (buffered tablet formulation) on the disposition of atevirdine, 12 human immunodeficiency virus type 1-infected subjects (mean CD4+ cell count, 199 cells per mm3; range, 13 to 447 cells/mm3) participated in a three-way, partially randomized, crossover, single-dose study to evaluate the pharmacokinetics of didanosine and atevirdine when each drug was given alone (treatments A and B, respectively) versus concurrently (treatment C). Concurrent administration of didanosine and atevirdine significantly reduced the maximum concentration of atevirdine in serum from 3.45 +/- 2.8 to 0.854 +/- 0.33 microM (P = 0.004). Likewise, the mean atevirdine area under the concentration-time curve from 0 to 24 h after administration of the combination was reduced to 6.47 +/- 2.2 microM.h (P = 0.004) relative to a value of 11.3 +/- 4.8 microM.h for atevirdine alone. Atevirdine had no statistically significant effect on the pharmacokinetic parameters of didanosine. Concurrent administration of single doses of atevirdine and didanosine resulted in a markedly lower maximum concentration of atevirdine in serum and area under the concentration-time curve, with a minimal effect on the disposition of didanosine. It is unknown whether an interaction of similar magnitude would occur under steady-state conditions; thus, combination regimens which include both atevirdine and didanosine should be designed so that their administration times are separated. Since the duration of the buffering effect of didanosine formulations is unknown, atevirdine should be given prior to didanosine.

Determination intestinal metabolism and permeability for several compounds in rats. Implications on regional bioavailability in humans

PURPOSE

To investigate the regional differences in small intestinal (SI) metabolism and permeability for several compounds and to ascertain the potential significance of these differences on the reported reductions in regional bioavailability in humans.

METHODS

The regional SI metabolism and permeability of captopril, didanosine (ddI), mannitol, ofloxacin and zidovudine (ZDV) were investigated in rats using a Single Pass Perfusion (SPIP) procedure or intestinal homogenates.

RESULTS

ddI was metabolized to a greater extent in the upper SI whereas captopril was metabolized to a greater extent in the lower SI. Relatively low homogenate concentrations resulted in significant degradation of captopril in the upper and lower SI. All other compounds were stable and changes in the buffer system or the initial concentration did not affect the results. The SI permeabilities of all compounds, with the exception of mannitol, decreased linearly with respect to SI location and the slopes of the corresponding normalized regression lines were not significantly different.

CONCLUSIONS

It has been reported that captopril and ddI demonstrate regional intestinal bioavailability in several species including humans. The current results suggest that the reported reduction in the lower SI bioavailability of captopril may be a result of a reduction in permeability and an increase in intestinal metabolism whereas for ddI, the reduction in the lower SI bioavailability appears to be attributable to a reduction in intestinal permeability. Other factors such as luminal metabolism may also significantly effect regional differences in the intestinal bioavailability of ddI or captopril. Based on these results, a strong possibility exists that ofloxacin and ZDV may also demonstrate regional differences in intestinal bioavailability.

Effects of cytokines on antiviral pharmacokinetics: an alternative approach to assessment of drug interactions using bioequivalence guidelines

The effects of cytokines on the pharmacokinetics of nucleoside analogs were evaluated in two separate studies using zidovudine in combination with interleukin-2 and didanosine in combination with alpha interferon. In each study, drug interactions were evaluated by using both a standard method (Student's t test) and bioequivalence testing. Serial blood samples were collected from human immunodeficiency virus-infected patients prior to and during cytokine therapy for determination of nucleoside analog concentrations. Concentrations were fit separately to a two-compartment model by using the iterative two-stage approach to population analysis. No alterations in area under the curve or oral clearance were observed for either drug during combination therapy. In general, there was good agreement between statistical methods for determining if antiviral pharmacokinetic parameters were altered by concomitant cytokine therapy. However, large individual changes in the maximum concentration of zidovudine in serum were detected by bioequivalence testing but no difference was found by Student's t test. For didanosine, significant but clinically irrelevant decreases determined by standard hypothesis testing were seen for both the volume of the central compartment (1.91 to 1.86 liters) and the absorption rate constant (0.79 to 0.73 h-1) in the presence of alpha interferon. No interaction was noted for these parameters by using bioequivalence guidelines. Bioequivalence testing may provide an alternative approach to assessment of drug interactions. Interleukin-2 and alpha interferon do not alter the pharmacokinetics of zidovudine and didanosine, respectively.