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.

Pharmacokinetics of the nonnucleoside reverse transcriptase inhibitor efavirenz among HIV-infected Ugandans

BACKGROUND

Pharmacokinetic variability of the nonnucleoside reverse transcriptase inhibitor efavirenz has been documented, and high variation in trough concentrations or clearance has been found to be a risk for virological failure. Africans population exhibits greater variability in efavirenz concentrations than other ethnic groups, and so a better understanding of the pharmacokinetics of the drug is needed in this population. This study characterized efavirenz pharmacokinetics in HIV-infected Ugandans.

METHODS

Efavirenz plasma concentrations were obtained for 66 HIV-infected Ugandans initiating efavirenz- based regimens, with blood samples collected at eight time-points over 24 h on day 1 of treatment, and at a further eight time-points on day 14. Noncompartmental analysis was used to describe the pharmacokinetics of efavirenz.

RESULTS

The mean steady-state minimum plasma concentration (C(min) ) of efavirenz was 2.9 µg/mL, the mean area under the curve (AUC) was 278.5 h µg/mL, and mean efavirenz clearance was 7.4 L/h. Although overall mean clearance did not change over the 2 weeks, 41.9% of participants showed an average 95.8% increase in clearance. On day 14, the maximum concentration (C(max) ) of efavirenz was >4 µg/mL in 96.6% of participants, while C(min) was <1 µg/mL in only 4.5%. Overall, 69% of participants experienced adverse central nervous system (CNS) symptoms attributable to efavirenz during the 2-week period, and 95% of these participants were found to have efavirenz plasma concentrations >4 µg/mL, although only half maintained a high concentration until at least 8 h after dosing.

CONCLUSION

The findings of this study show that HIV-infected patients on efavirenz may exhibit autoinduction to various extents, and this needs to be taken into consideration in the clinical management of individual patients. Efavirenz CNS toxicity during the initial phase of treatment may be related to C(max) , regardless of the sampling time.

Update on the Pharmacokinetic Aspects of Antiretroviral Agents: Implications in Therapeutic Drug Monitoring

The observed inter-individual variation in antiretroviral pharmacokinetics (PK) that results in a wide range of drug exposures from fixed-dose regimens has led to increasing interest in the clinical use of therapeutic drug monitoring (TDM) to individualize dosing of antiretroviral therapy (ART). The focus of this review is to provide an overview of literature available to support therapeutic drug monitoring among the current classes of antiretrovirals, suggest patient populations that may benefit from TDM and bring forth some of the limitations that may exist for widespread use of TDM in a traditional clinical setting.

Clinical pharmacokinetics of non-nucleoside reverse transcriptase inhibitors

Non-nucleoside reverse transcriptase inhibitors (NNRTIs) are a diverse group of compounds that induce allosteric changes in the human immunodeficiency virus type 1 (HIV-1) reverse transcriptase, thus rendering the enzyme incapable of converting viral RNA to DNA. Unlike nucleoside analogue inhibitors of reverse transcriptase, NNRTIs do not require sequential phosphorylation to elicit antiretroviral activity. There are currently 3 approved NNRTIs: nevirapine, delavirdine and efavirenz. Although possessing a common mechanism of action, these agents can be differentiated by both molecular and pharmacokinetic characteristics. Each of the NNRTIs is metabolised to some degree by the cytochrome P450 (CYP) system of enzymes, making them prone to clinically significant drug interactions. In addition, they elicit variable effects on other medications, acting as either inducers or inhibitors of drugs metabolised by CYP. These drug interactions are an important consideration in the clinical use of these agents as a part of combination antiretroviral therapy. Additional factors such as the influence of food and pH on oral absorption, and protein binding, must also be considered.

Once-a-day Concerta methylphenidate versus three-times-daily methylphenidate in laboratory and natural settings

OBJECTIVE

Methylphenidate (MPH), the most commonly prescribed drug for attention-deficit/hyperactivity disorder (ADHD), has a short half-life, which necessitates multiple daily doses. The need for multiple doses produces problems with medication administration during school and after-school hours, and therefore with compliance. Previous long-acting stimulants and preparations have shown effects equivalent to twice-daily dosing of MPH. This study tests the efficacy and duration of action, in natural and laboratory settings, of an extended-release MPH preparation designed to last 12 hours and therefore be equivalent to 3-times-daily dosing.

METHODS

Sixty-eight children with ADHD, 6 to 12 years old, participated in a within-subject, double-blind comparison of placebo, immediate-release (IR) MPH 3 times a day (tid), and Concerta, a once-daily MPH formulation. Three dosing levels of medication were used: 5 mg IR MPH tid/18 mg Concerta once a day (qd); 10 mg IR MPH tid/36 mg Concerta qd; and 15 mg IR MPH tid/54 mg Concerta qd. All children were currently medicated with MPH at enrollment, and each child's dose level was based on that child's MPH dosing before the study. The doses of Concerta were selected to be comparable to the daily doses of MPH that each child received. To achieve the ascending rate of MPH delivery determined by initial investigations to provide the necessary continuous coverage, Concerta doses were 20% higher on a daily basis than a comparable tid regimen of IR MPH. Children received each medication condition for 7 days. The investigation was conducted in the context of a background clinical behavioral intervention in both the natural environment and the laboratory setting. Parents received behavioral parent training and teachers were taught to establish a school-home daily report card (DRC). A DRC is a list of individual target behaviors that represent a child's most salient areas of impairment. Teachers set daily goals for each child's impairment targets, and parents provided rewards at home for goal attainment. Each weekday, teachers completed the DRC, and it was used as a dependent measure of individualized medication response. Teachers and parents also completed weekly standardized ratings of behavior and treatment effectiveness. To evaluate the time course of medication effects, children spent 12 hours in a laboratory setting on Saturdays and medication effects were measured using procedures and methods adapted from our summer treatment program. Measures of classroom behavior and academic productivity/accuracy were taken in a laboratory classroom setting during which children completed independent math and reading worksheets. Measures of social behavior were taken in structured, small-group board game settings and unstructured recess settings. Measures included behavior frequency counts, academic problems completed and accuracy, independent observations, teacher and counselor ratings, and individualized behavioral target goals. Reports of adverse events, sleep quality, and appetite were collected.

RESULTS

On virtually all measures in all settings, both drug conditions were significantly different from placebo, and the 2 drugs were not different from each other. In children's regular school settings, both medications improved behavior as measured by teacher ratings and individualized target behaviors (the DRC); these effects were seen into the evening as measured by parent ratings. In the laboratory setting, effects of Concerta were equivalent to tid MPH and lasted at least through 12 hours after dosing. Concerta was significantly superior to tid MPH on 2 parent rating scores, and when asked, more parents preferred Concerta than preferred tid IR MPH or placebo. Side effects on children's sleep and appetite were similar for the 2 preparations. In the lab setting, both medications improved productivity and accuracy on arithmetic seatwork assignments, disruptive and on-task behavior, and classroom rule following. Both medications improved children's rule following and negative behavior in small group board games, as well as in unstructured recess settings. Individual target behaviors also showed significant improvement with medication across domains in the laboratory setting. Children's behavior across settings deteriorated across the laboratory day, and the primary effect of medication was to prevent this deterioration as the day wore on. Results support the use of background behavioral treatment in clinical trials of stimulant medication, and illustrate the utility of a measure of individualized daily target goals (ie, the DRC) as an objective measure of medication response in both the laboratory and natural school settings.

CONCLUSION

This investigation clearly supports the efficacy of the Concerta long-acting formulation of MPH for parents who desire to have medication benefits for their child throughout the day and early evening. (ABSTRACT TRUNCATED)

Glucose disorders associated with HIV and its drug therapy

OBJECTIVE

To review the impact that factors such as HIV infection, antiretrovirals, and other commonly used drug therapies have on glucose metabolism in HIV-infected patients.

DATA SOURCES

Pertinent literature was identified via a MEDLINE search from 1980 to April 2000 and through secondary sources (abstracts presented at recent scientific meetings, manufacturers' package inserts). The key words used were antiretroviral therapy, HIV infection, insulin resistance, and metabolic abnormalities. All information deemed relevant to evaluate the impact that HIV infection and drug therapy have on glucose metabolism in HIV-infected patients was included.

DATA SYNTHESIS

The viral burden and stress that are present in HIV-infected patients elicit a complex hormonal and immunologic response that may alter various biochemical pathways, including glucose metabolism. Although rare before the era of potent antiretroviral therapy, insulin resistance has now been described as an important component of the lipodystrophy syndrome. The complex and multifactorial nature of glucose metabolism dysregulation makes management of hyperglycemia or diabetes mellitus challenging in HIV-infected patients. In such a context, a set of recommendations was developed to guide practitioners in assessing, treating, and monitoring hyperglycemia or diabetes mellitus in HIV-infected patients.

CONCLUSIONS

Alterations of glucose metabolism observed in HIV-infected patients are more frequent since the introduction of potent antiretroviral therapy. Although the etiology of such abnormalities remains unknown, protease inhibitors and, to a lesser extent, nucleoside reverse transcriptase inhibitors are believed to participate in their pathogenic mechanisms. Because of similarities to the pathogenesis of diabetes mellitus, management of antiretroviral-induced hyperglycemia could follow that the recommendations of the American Diabetes Association, with special considerations for monitoring patients with HIV infection. Future studies of altered glucose metabolism in HIV-infected patients should focus on understanding the precise mechanism or causes of this complication so that preventive and therapeutic guidelines can be further evaluated.

Concentration-targeted phase I trials of atevirdine mesylate in patients with HIV infection: dosage requirements and pharmacokinetic studies. The ACTG 187 and 199 study teams

RATIONALE

To determine the dosage requirements and pharmacokinetics of atevirdine, a non-nucleoside reverse transcriptase inhibitor and its N-dealkylated metabolite (N-ATV) during phase I studies in patients receiving atevirdine alone or in combination with zidovudine.

DESIGN

Two open label, phase I studies conducted by the adult AIDS Clinical Trials Group (ACTG) in which atevirdine was administered every 8 h with weekly dosage adjustments to attain targeted trough plasma atevirdine concentrations.

SETTING

Five Adult AIDS Clinical Trials Units.

PATIENTS

Fifty patients (ACTG 199; n = 20 and ACTG 187; n = 30) with HIV-1 infection and < or =500 CD4+ lymphocytes/mm3.

INTERVENTION

ACTG 199; 12 weeks of therapy with atevirdine (dose-adjusted to achieve plasma trough atevirdine concentrations of 5-10 microM) and zidovudine (200 mg every 8 h). ACTG 187: 12 weeks of atevirdine monotherapy with atevirdine doses adjusted to achieve escalating, targeted trough plasma concentration ranges (5-13, 14-22, and 23-31 microM).

MEASUREMENTS

ACTG 199: atevirdine, N-ATV and zidovudine trough determinations weekly (all patients) and intensive pharmacokinetics (selected patients) prior to and at 6 and 12 weeks during combination therapy. ACTG 187: atevirdine and N-ATV trough concentrations over a 12 week period. Intensive pharmacokinetic studies were conducted prior to and at 4 and/or 8 weeks during atevirdine monotherapy in female patients.

RESULTS

Atevirdine plasma concentrations demonstrated considerable interpatient variability which was minimized by the adjustment of maintenance doses (range: 600-3900 mg/day) to achieve the desired trough concentrations. In ACTG 187, the mean number of weeks to attain the target value, and the percentage of patients who attained the target, was group I (5-11 microM): 2.7+/-2.4 weeks (92%); group II (12-21 microM): 2.6+/-1.8 (64%); and group III (22-31 microM): 7.0+/-5.6 weeks (27%). In ACTG 199 it was 3.2+/-5.2 weeks (95%) to achieve a 5-10 microM trough. Atevirdine demonstrated a mono- or bi-exponential decline among most of the patients studied after the first dose. During multiple-dosing a number of patterns of atevirdine disposition were observed including; rapid absorption with Cmax at 0.5-1 h, delayed absorption with Cmax at 3-4 h; minimal Cmax to Cmin fluctuation and Cmax to Cmin ratios of > 4. N-ATV (an inactive metabolite) patterns were characterized on day one by rapid appearance of the metabolite which peaked at 2-3 h after the dose and declined in a mono- or bi-exponential manner. At steady-state N-ATV patterns demonstrated minimal Cmax to Cmin fluctuations with some of the patients having more stable plasma N-ATV concentrations, while others had greater fluctuations week to week.

CONCLUSIONS

Considerable interpatient variability was noted in the pharmacokinetics of atevirdine. The variation in drug disposition was reflected in the range of daily doses required to attain the targeted trough concentrations. Atevirdine metabolism did not appear to reach saturation during chronic dosing in many of our patients, as reflected by the pattern of N-ATV/ATV ratios in plasma and saturation was not an explanation for the variation in dosing requirements. No apparent differences were noted between males and females, and atevirdine did not appear to influence zidovudine disposition.

Accuracy of measured vancomycin serum concentrations in patients with end-stage renal disease

OBJECTIVE

To review information related to the accuracy of vancomycin serum drug concentrations in patients with end-stage renal disease, focusing on available assays and mechanisms of cross-reactivity.

DATA SOURCES

Primary and review articles identified from a MEDLINE search (January 1980-June 1999) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All articles identified were evaluated, and all relevant information was included in this review.

DATA SYNTHESIS

Falsely elevated vancomycin serum concentrations may occur in patients with renal dysfunction. The underlying mechanism is due to the formation and accumulation of a pseudo-metabolite, the vancomycin crystalline degradation product (CDP). Vancomycin is converted to CDP when exposed to heat, including normal body temperature. Because the molecular structures of CDP and vancomycin are similar, both molecules are detected by polyclonal immunoassay systems used in clinical laboratories. This cross-reactivity leads to falsely elevated serum vancomycin concentrations in excess of 50-70%. Such large assay inaccuracies may result in improper dosage adjustments and therapeutic failures. A monoclonal immunoassay system has been developed that does not significantly cross-react with CDP.

CONCLUSIONS

To appropriately interpret laboratory results, it is essential for clinicians to be aware of the vancomycin-CDP cross-reactivity problem and to be familiar with the specific assay used to measure vancomycin concentrations in patients with renal dysfunction.

Comparison of a neural network approach with five traditional methods for predicting creatinine clearance in patients with human immunodeficiency virus infection

STUDY OBJECTIVE

To compare the results of an artificial neural network approach with those of five published creatinine clearance (Cl(cr)) prediction equations and with the measured (true) Cl(cr) in patients infected with the human immunodeficiency virus (HIV).

DESIGN

Six-month prospective study.

SETTINGS

Two university medical centers.

PATIENTS

Sixty-five HIV-infected patients: 18 relatively healthy outpatients and 47 inpatients.

INTERVENTIONS

All subjects had urine collected for 24 hours to determine Cl(cr).

MEASUREMENTS AND MAIN RESULTS

The 16 input variables were age, ideal body weight, actual body weight, body surface area, height, and the following blood chemistries: sodium, potassium, aspartate aminotransferase, alanine aminotransferase, red blood cell count, platelet count, white blood cell count, glucose, serum creatinine, blood urea nitrogen, and albumin. The only output variable was Cl(cr). A training set of 55 subjects was used to develop the relationship between input variables and the output variable. The trained neural network was then used to predict Cl(cr) of a validation set of 10 subjects. Mean differences between predicted Cl(cr) and actual Cl(cr) (bias) were 4.1, 28.7, 29.4, 26.0, 31.8, and 55.8 ml/min/1.73 m2 for the artificial neural network, Cockcroft and Gault, Jelliffe 1, Jelliffe 2, Mawer et al, and Hull et al methods, respectively.

CONCLUSION

The accuracy of predicting Cl(cr) in subjects with HIV infection by the artificial neural network is superior to that of the five equations that are currently used in clinical settings.

Previous infection with Helicobacter pylori is the primary determinant of spontaneous gastric hypoacidity in human immunodeficiency virus-infected outpatients

To investigate the incidence and demographics of gastric hypoacidity among persons infected with human immunodeficiency virus (HIV), 146 asymptomatic subjects were evaluated with use of a radiotelemetry device (Heidelberg capsule). Gastric hypoacidity (minimum gastric pH of > or = 3) occurred in 24 subjects (17%). Demographic characteristics, CD4 cell counts, and Helicobacter pylori serological status were evaluated for an association with gastric pH. Subjects with hypoacidity were more likely to have positive H. pylori serology than were subjects without hypoacidity (15 of 24 vs. 23 of 74, respectively; P = .004). Multivariate analysis indicated that a positive H. pylori serology was the most significant predictor of hypoacidity, accounting for an increase in gastric pH of 39%. A history of injection drug use, heterosexual transmission of HIV, and male gender were also associated with an elevated gastric pH. CD4 cell counts did not contribute to predictions of gastric pH. A history of H. pylori infection is relatively common in HIV-positive black and Hispanic populations and is a predictor of gastric pH.

Relationship between didanosine exposure and surrogate marker response in human immunodeficiency virus-infected outpatients

We used information available from routine clinic visits to characterize the pharmacokinetics of didanosine in 82 human immunodeficiency virus-infected patients. A total of 271 blood samples were collected for the measurement of didanosine concentrations in plasma (mean +/- standard deviation [SD], 3.30 +/- 2.21 samples/patient). Bayesian estimates of didanosine oral clearance (CL[oral]) were obtained for these patients by the POSTHOC option within the NONMEM software package. Population priors from a previous NONMEM analysis of didanosine pharmacokinetics were used. The mean +/- SD CL(oral) was 132 +/- 27.7 liters/h, which agrees reasonably well with estimates obtained from previous pharmacokinetic studies of didanosine. Estimates of individual didanosine exposure were then used to consider potential relationships between drug exposure and surrogate marker response over a 6-month period. No correlations were found between the didanosine area under the concentration-time curve from 0 to 6 months and the absolute CD4 cell count (r = 0.305; 0.1 < P < 0.2), weight response (r = 0.0857; P > 0.4), or percentage of CD4 lymphocytes (r = 0.0559; P > 0.4). Future efforts to characterize didanosine exposure in outpatients by random sampling methods should involve more directed efforts to limit residual variability in the data.

Zalcitabine population pharmacokinetics: application of radioimmunoassay

Zalcitabine population pharmacokinetics were evaluated in 44 human immunodeficiency virus-infected patients (39 males and 5 females) in our immunodeficiency clinic. Eighty-one blood samples were collected during routine clinic visits for the measurement of plasma zalcitabine concentrations by radioimmunoassay (1.84+/-1.24 samples/patient; range, 1 to 6 samples/patient). These data, along with dosing information, age (38.6+/-7.13 years), sex, weight (79.1+/-15.0 kg), and estimated creatinine clearance (89.1+/-21.5 ml/min), were entered into NONMEM to obtain population estimates for zalcitabine pharmacokinetic parameters. The standard curve of the radioimmunoassay ranged from 0.5 to 50.0 ng/ml. The observed concentrations of zalcitabine in plasma ranged from 2.01 to 8.57 ng/ml following the administration of doses of either 0.375 or 0.75 mg. A one-compartment model best fit the data. The addition of patient covariates did not improve the basic fit of the model to the data. Oral clearance was determined to be 14.8 liters/h (0.19 liter/h/kg; coefficient of variation [CV] = 23.8%), while the volume of distribution was estimated to be 87.6 liters (1.18 liters/kg; CV = 54.0%). We were also able to obtain individual estimates of oral clearance (range, 8.05 to 19.8 liters/h; 0.11 to 0.30 liter/h/kg) and volume of distribution (range, 49.2 to 161 liters; 0.43 to 1.92 liters/kg) of zalcitabine in these patients with the POSTHOC option in NONMEM. Our value for oral clearance agrees well with other estimates of oral clearance from traditional pharmacokinetic studies of zalcitabine and suggests that population methods may be a reasonable alternative to these traditional approaches for obtaining information on the disposition of zalcitabine.

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.

Radioimmunoassay of zidovudine: extended use and potential application

When first approved, the dosing regimens for zidovudine were 1,200-1,500 mg/day; however, because toxicity developed, the daily dose had to be reduced to 500-600 mg/day. At these lower doses, plasma concentrations for a considerable segment of the dosing interval are often below the assay sensitivity for the high-performance liquid chromatography (HPLC) method. Although commonly used, the zidovudine radioimmunoassay has had minimal documentation for the quantitative analysis of clinical samples, especially at current doses. The authors' findings indicate that plasma, urine treated with phosphate buffer, and cerebrospinal fluid samples may be assayed using a commercially available radioimmunoassay. A good correlation was found for clinical samples measured by radioimmunoassay and HPLC (R2 = 0.85). The greater assay sensitivity, ability to process multiple specimens, and the relatively rapid turnaround time suggest that the zidovudine radioimmunoassay may have an important role in clinical trials evaluating zidovudine pharmacokinetics. This report summarizes the authors' experience with the zidovudine radioimmunoassay and focuses on its potential use in studying the role of therapeutic drug monitoring for zidovudine.

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.

In vitro protein-binding characteristics of delavirdine and its N-dealkylated metabolite

This study was performed to determine delavirdine protein-binding characteristics as well as those of its N-dealkylated metabolite (N-DLV). Initial studies of 36 microM delavirdine and 30 microM N-DLV in solutions of plasma, albumin 4 g%, alpha-1-acid glycoprotein (AAG) 100 mg% or immune globulin (IVIG) 5 g% were conducted. Delavirdine (12, 36 and 73 microM) and N-DLV (10, 30 and 60 microM) were then studied alone and in combination in plasma and various concentrations of albumin. Studies were done in triplicate using equilibrium dialysis. The mean delavirdine fraction unbound (fu) in plasma, albumin, IVIG and AAG was 0.013, 0.033, 0.752 and 0.912 while the mean fu of N-DLV in these same protein solutions was 0.139, 0.195, 0.329 and 0.359. In plasma and albumin, a greater fu was observed at higher delavirdine concentrations and no significant changes in fu were noted with the addition of N-DLV. An increase in delavirdine fu was noted as the albumin concentrations decreased. The fu of N-DLV increased significantly as the concentration of albumin decreased as well as with decreasing N-DLV concentration. The potential implications of extensive delavirdine binding to plasma proteins, primarily albumin, are discussed.

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.

Comparison of measured and estimated creatinine clearance in patients with advanced HIV disease

STUDY OBJECTIVE

To assess the accuracy of five creatinine clearance equations in predicting measured creatinine clearance in hospitalized patients with human immunodeficiency viral (HIV) infection.

DESIGN

Prospective evaluation over a 6-month period.

SETTING

Erie County Medical Center, a 550-bed teaching institution.

PATIENTS

Forty-seven HIV-positive patients (39 men, 8 women) who were admitted for a variety of HIV-related illnesses and judged clinically to have stable renal function. Of the 47 original patients, 44 were evaluable based on exclusion criteria.

INTERVENTIONS

Serum creatinine and 24-hour measured creatinine clearance were performed in each patient.

MEASUREMENTS AND MAIN RESULTS

The estimated creatinine clearance from each of the equations (Cockcroft-Gault, two Jeliffe equations, Mawer et al, and Hull et al) was compared with the measured creatinine clearance. Statistical analysis of these comparisons was performed and all of the equations were found to overestimate the measured creatinine clearance (mean error 34-45%).

CONCLUSIONS

Many HIV-infected patients have a decreased creatinine clearance despite a serum creatinine concentration within the normal range. Each of the equations overestimated the measured creatinine clearance.

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.

Pharmacokinetics of zidovudine and didanosine during combination therapy

While the combination of zidovudine and didanosine is used in HIV-infected patients with more advanced disease, the possibility of a pharmacokinetic interaction between these two drugs remains controversial. Zidovudine doses of 50, 100, and 200 mg, combined with 67, 167, and 250 mg of didanosine were evaluated in 11 asymptomatic HIV-infected patients after receiving 24 weeks of combination therapy in AIDS Clinical Trials Group protocol 143. The pharmacokinetic parameters of zidovudine and didanosine were similar to those obtained with each drug given as monotherapy in other previously published studies. The renal clearance and urinary recovery of glucuronidated zidovudine was reduced when zidovudine was given in combination with didanosine, possibly due to competition for renal tubular secretion. These data suggest that no clinically important pharmacokinetic interaction occurs when zidovudine and didanosine are given together.

Phase I study of atevirdine, a nonnucleoside reverse transcriptase inhibitor, in combination with zidovudine for human immunodeficiency virus type 1 infection. ACTG 199 Study Team

Twenty patients were enrolled in a phase I clinical trial of atevirdine, a nonnucleoside reverse transcriptase inhibitor (NNRTI), given in combination with zidovudine for treatment of human immunodeficiency virus type 1 (HIV-1) infection. Fifteen patients had received no previous antiretroviral therapy. HIV-1 isolates obtained at 6-week intervals were tested for sensitivity to atevirdine and zidovudine. Two patients developed a rash within 2 weeks of enrollment, and 1 of these developed concomitant fever and hepatitis. No hematopoietic, neurologic, or pancreatic toxicities were observed. Atevirdine had considerable initial interpatient pharmacokinetic variability. Forty-seven percent of patients treated with atevirdine plus zidovudine had increased CD4 lymphocyte counts, and HIV isolates from 62% of patients remained sensitive to atevirdine after 24 weeks of therapy. Atevirdine plus zidovudine was well-tolerated. Additional studies should be done to determine the role of atevirdine in the therapy for HIV infection.

Evidence of alkaline phosphatase interference in a zidovudine radioimmunoassay

Phosphorylated zidovudine (ZDV) concentrations may provide a link between drug exposure and clinical efficacy since these would include the active, intracellular form of the drug, ZDV triphosphate. Many groups are investigating the optimal methodology that can be used to accomplish this goal. The initial purpose of the present studies was to examine the effect of the inclusion of cell wash steps on the quantitation of intracellular ZDV. Ten milliliters of whole blood collected from healthy volunteers was spiked with increasing ZDV concentrations (0.187, 0.375, 1.87, and 3.75 microM), allowed to equilibrate at room temperature for 1 h, and separated into whole-blood components by a density gradient procedure. A mononuclear cell pellet was obtained, reconstituted with 2 ml of phosphate-buffered saline (PBS), and split into two aliquots, one of which was not washed at all and the other of which was washed four times with 1 ml of PBS. All samples were analyzed by ZDV radioimmunoassay (RIA) after a 1:1 dilution with either 1 mg of alkaline phosphatase (type 1-S; Sigma) per ml or PBS. Parent ZDV was measured in those samples which were not treated with the enzyme, while total ZDV was measured in those samples which were exposed to alkaline phosphatase (21 degrees C for 1 h). The result of the difference between the two samples is total phosphorylated ZDV. During the experiment, evidence of alkaline phosphatase interference with the RIA became apparent, confusing interpretation of intracellular ZDV concentrations. This evidence was based on three sets of data. First, wash samples showed increases in ZDV concentrations of as great as 0.127 microgramM after exposure to alkaline phosphatase, even though on microscopic inspection the wash samples were acellular. Second, the sum of total ZDV recovered from the four wash samples plus the washed cell pellet was as much as 14-fold greater than the total ZDV measured in the unwashed cell pellet. Theoretically, at least, these two entities should be equal. Finally, control samples of alkaline phosphatase in PBS (0.5 mg/ml) run directly through the assay measured false ZDV levels ranging from 0.002 to 0.075 microgramM (0.6 to 20 ng/ml). Alkaline phosphatase is frequently used to measure phosphorylated anabolites of ZDV in peripheral blood mononuclear cells. These data show that the particular form of alkaline phosphatase used may interfere with the ZDV RIA and may confuse the interpretation of phosphorylated anabolite concentrations of ZDV.

In vitro protein-binding characteristics of atevirdine and its N-dealkylated metabolite

The in vitro protein-binding characteristics of atevirdine (ATV), a non-nucleoside reverse transcriptase inhibitor with activity against HIV-1, and its N-dealkylated metabolite (N-ATV) were studied using equilibrium dialysis. ATV and N-ATV were studied at concentrations of 5, 10, 20, and 30 microM in five protein-containing solutions [albumin 4%, plasma, serum, immune globulin (IgG) 1.5%, alpha 1-acid glycoprotein (AAG)] for 5 h at 37 degrees C. All samples were analyzed by high-performance liquid chromatography. The free fraction of atevirdine in plasma, albumin, and serum was 0.01-0.02 over the range of drug concentrations studied. The fraction unbound (fu) in these protein solutions statistically differed from IgG and AAG (P < 0.05), where the fraction unbound averaged 0.96 and 0.53, respectively. N-ATV had a similar binding profile as ATV with a fraction unbound of 0.04, 0.03, 0.03 in albumin, plasma and serum, respectively. A difference existed in N-ATV binding when compared to IgG and AAG with an average fu of 0.87 and 0.59 (P < 0.05 vs. plasma). The potential clinical implications of the high degree of protein binding for ATV and N-ATV are discussed.

Prolonged, but not diminished, zidovudine absorption induced by a high-fat breakfast

STUDY OBJECTIVE

To determine the effect of a high-fat breakfast on single-dose, zidovudine (ZDV) pharmacokinetics.

DESIGN

Open-label, randomized, crossover study.

PATIENTS

Eighteen asymptomatic subjects (12 men, 6 women) infected with the human immunodeficiency virus (mean CD4 cell counts of 512 +/- 178/mm3).

INTERVENTIONS

Subjects received single 100-mg oral doses of ZDV as follows: after an 8-hour fast (treatment A), with a high-fat breakfast (treatment B), and 3 hours after a high-fat breakfast (treatment C).

MEASUREMENTS AND MAIN RESULTS

The high-fat breakfast significantly reduced the mean (coefficient of variation) maximum plasma concentration (Cmax) from 806 (55%) ng/ml with treatment A to 341 (47%) and 424 (42%) ng/ml with treatments B and C, respectively. The time to Cmax was significantly prolonged from 0.68 (30%) hours with treatment A to 1.7 (54%) and 1.3 (42%) hours with treatments B and C, respectively. Area under the plasma ZDV concentration-time curve (AUC) was not statistically different across the study treatments. Men had significantly lower (35%) renal clearances of both ZDV and its glucuronide metabolite than women.

CONCLUSIONS

When ZDV was given either with or 3 hours after a high-fat breakfast, its absorption was prolonged and Cmax was reduced relative to fasting. However, systemic exposure, as indicated by AUC, was unchanged.

Mapping of bactericidal epitopes on the P2 porin protein of nontypeable Haemophilus influenzae

The P2 porin protein is the major outer membrane protein of nontypeable Haemophilus influenzae and is a potential target of a protective immune response. Nine monoclonal antibodies (MAbs) to P2 were developed by immunizing mice with nontypeable H. influenzae whole organisms. Each MAb reacted exclusively with the homologous strain in a whole-cell immunodot assay demonstrating exquisite strain specificity. All nine MAbs recognized abundantly expressed surface-exposed epitopes on the intact bacterium by immunofluorescence and immunoelectron microscopy. Each MAb was bactericidal to the homologous strain in an in vitro complement-mediated killing assay. Immunoblot assay of cyanogen bromide cleavage products of purified P2 indicated that MAb 5F2 recognized the 10-kDa fragment, and the other eight MAbs recognized the 32-kDa fragment. Competitive ELISAs confirmed that 5F2 recognized an epitope that is different from the other eight MAbs. To further localize epitopes, MAbs 5F2 and 6G3 were studied in protein footprinting by using reversed-phase high-performance liquid chromatography. Three potential epitope-containing peptides which were reactive in an enzyme-linked immunosorbent assay with both 5F2 and 6G3 were isolated. These peptides were identified by N-terminal amino acid sequence and localized to loops 5 and 8 of the proposed model for P2. Fusion proteins consisting of glutathione S-transferase fused with variable-length peptides from loops 5 and 8 were expressed in the pGEX-2T vector. Immunoblot assay of fusion peptides of loops 5 and 8 confirmed that 5F2 recognized an epitope within residues 338 to 354 of loop 8; 6G3 and the remaining MAbs recognized an epitope within residues 213 to 229 of loop 5. These studies indicate that nontypeable H. influenzae contains bactericidal epitopes which have been mapped to two different surface-exposed loops of the P2 molecule. These potentially protective epitopes are strain specific and abundantly expressed on the surface of the intact bacterium.

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.

Disposition of intramuscular cefonicid in elderly patients

OBJECTIVE

To examine the disposition of intramuscular (IM) cefonicid in elderly patients with bacterial pneumonia.

DESIGN

Pharmacokinetic study.

SETTING

A 620-bed university-affiliated long-term care institution with its own 39-bed acute care unit.

PATIENTS

Nine consecutive elderly patients with bacterial pneumonia treated with IM cefonicid.

MEASUREMENTS

Blood samples were collected on the seventh day of therapy over a 24-hour period and analyzed by high performance liquid chromatography. Pharmacokinetics parameters (volume of distribution, half-life, and clearance) and protein binding were calculated. Clinical outcome of IM cefonicid therapy was also noted.

RESULTS

The estimated creatinine clearance (CIcr) ranged from 32 to 145 mL/min. Peak cefonicid serum concentrations occurred at 0.5-1.5 hours, with a mean value of 118 +/- 41 micrograms/mL. Cefonicid concentrations declined monoexponentially to 57 +/- 16 micrograms/mL at 12 hours and 28 +/- 14 micrograms/mL at 24 hours. The mean apparent distribution volume was 0.2 +/- 0.07 L/kg, and the mean apparent total clearance was 15 +/- 12 mL/min. The half-life ranged from 3.1 to 38 hours. A linear correlation was noted between Clcr and cefonicid clearance (r = 0.99).

CONCLUSIONS

Cefonicid absorption was variable among these patients, and the serum half-life was longer than previous values noted in younger patients with similar degree of renal dysfunction. Pharmacokinetic and clinical outcome data from our study group indicate the potential role of IM cefonicid in treating elderly patients with bacterial pneumonia.

Intravascular distribution of zidovudine: role of plasma proteins and whole blood components

Knowledge of drug protein-binding and blood cell partitioning may be important for evaluating the pharmacokinetic parameters of zidovudine, particularly because of its intracellular site of action and potential to induce side effects. Equilibrium dialysis studies of zidovudine were performed over 2 h to identify the extent and site of binding. Zidovudine was added to anticoagulated whole blood to study blood cell distribution over a 24 h period at 37 degrees C and at 21 degrees C. Concurrent plasma and whole blood samples were determined at various time-points and blood partitioning was determined by application of a mass balance equation. All samples were analyzed using radioimmunoassay. The free fraction of zidovudine at a concentration of 500 ng/ml (1.7 microM) was 0.77 +/- 0.05 in plasma, 0.78 +/- 0.03 in serum, 0.88 +/- 0.03 in 4 g/dl albumin solution, and 1.0 in 100 mg/dl alpha 1-acid glycoprotein solution. A free fraction of 0.72 +/- 0.10 was observed in plasma from HIV-infected patients with zidovudine concentrations ranging from 16 to 91 ng/ml. Zidovudine equilibration between plasma and blood cells occurred rapidly, being complete within 10 min. After equilibrium was complete, the mean whole blood:plasma ratio was 0.86 +/- 0.02 and 0.80 +/- 0.04 (P = 0.20) and mean blood cell Partitioning ratio, [cell]/[plasma-free], was 0.85 +/- 0.06 and 0.66 +/- 0.14 (P = 0.25) for studies at 37 degrees C and 21 degrees C, respectively. The partitioning ratio was relatively consistent over the study period, suggesting no accumulation in blood cells. These results suggest that zidovudine binds to a small extent primarily to albumin. The free concentration equilibrates readily between blood cells and plasma independent of concentration and without signs of accumulation.

Zidovudine pharmacokinetics in HIV-positive women during different phases of the menstrual cycle

STUDY OBJECTIVE

To examine the pharmacokinetics of zidovudine during the menstrual cycle in human immunodeficiency virus- (HIV-) positive women.

DESIGN

Open, unblinded study.

SETTING

A women's clinic for acquired immunodeficiency syndrome (AIDS) at a large medical center.

PATIENTS

HIV-positive women with a CD4+ cell count above 200/mm3, receiving long-term zidovudine therapy, with a history of regular menstrual cycles.

INTERVENTIONS

All patients received a 100-mg dose of zidovudine in the fasted state on three occasions.

MEASUREMENTS AND MAIN RESULTS

Zidovudine and zidovudine-glucuronide plasma concentrations were measured with radioimmunoassay to determine the pharmacokinetic characteristics during each menstrual phase. The drug's mean peak plasma concentrations (range 233-808 ng/ml) were 556 +/- 145, 385 +/- 132, and 495 +/- 143 ng/ml during the menstrual, late follicular-ovulatory, and luteal phases, respectively. Initially, plasma concentrations declined in a linear fashion from 0 to 4 hours, with a prolonged elimination phase in many patients after 4 hours. The mean zidovudine area under the curve was 886 +/- 156, 845 +/- 268, and 775 +/- 167 ng.hour/ml. The mean percentage of dose recovered was 44.2 +/- 26.0, 56.9 +/- 19.1, and 42.2 +/- 16.6, respectively.

CONCLUSIONS

The pharmacokinetics of zidovudine were not different during the three phases of the menstrual cycle; however, considerable intrapatient and interpatient variability was noted for many of the values.

Zalcitabine

OBJECTIVE

To review the chemistry, intracellular metabolism, pharmacokinetics, and clinical trials of zalcitabine (2'3'-dideoxycytidine, ddC).

DATA SOURCES

English-language articles and conference procedings. The indexing terms used were zalcitabine, 2'3'-dideoxycytidine, and ddC.

STUDY SELECTION

In addition to the manufacturer's package insert, available Phase I and Phase I/II studies were reviewed.

DATA EXTRACTION

Clinical experience with ddC has been limited to uncontrolled studies and an expanded-access program. Efficacy was evaluated solely on surrogate markers of HIV disease: CD4+ lymphocyte counts and p24 antigen determinations. Clinical endpoints, such as disease progression and survival rates, must be provided to the Food and Drug Administration (FDA) for continued approval.

DATA SYNTHESIS

The FDA has approved use of ddC in combination with zidovudine (ZDV) as therapy of HIV infection for patients with CD4+ lymphocyte counts < or = 300 cells/mm3 who have experienced significant clinical or immunologic deterioration. Although ddC has the same mechanism of action as other nucleoside analogs, it is more potent on a molar basis. The drug is stable in gastric pH and has good bioavailability (approximately 70-90 percent), but is rapidly cleared from plasma (half life approximately 1-3 h). Intracellular concentrations of ddC triphosphate, the active form, are probably related to plasma concentrations, yet may persist in cells longer than the parent drug persists in plasma. When used as primary therapy in patients with CD4+ < or = 300 cells/mm3, ddC/ZDV increased CD4+ lymphocyte counts and reduced plasma p24 antigen concentrations. In comparison to ZDV monotherapy data taken from other studies, ddC/ZDV appeared to demonstrate a more pronounced and sustained increase in CD4+ cell counts; however, this observation cannot be confirmed until the results of ZDV-controlled comparisons are available. Overall, 17-31 percent of the patients receiving the currently recommended initial dosage of ddC experience peripheral neuropathy.

CONCLUSIONS

In combination with ZDV, ddC appears to augment the CD4+ cell response of ZDV monotherapy in the treatment of HIV infection for ZDV-naive patients, although controlled studies and rigorous statistical analyses are lacking at present. The efficacy of ddC/ZDV in patients who received prior treatment with ZDV monotherapy is unclear at the present.

Pharmacokinetics and pharmacodynamics of granulocyte-macrophage colony-stimulating factor and zidovudine in patients with AIDS and severe AIDS-related complex

Granulocytopenia is a complication of human immunodeficiency virus disease, as well as a toxic manifestation of zidovudine therapy. To evaluate pharmacokinetic and pharmacodynamic relationships, 11 AIDS-AIDS-related complex patients who had developed zidovudine-associated granulocytopenia (mean absolute neutrophil count, 1,077/mm3) were examined after addition of granulocyte-macrophage colony-stimulating factor (GM-CSF) to zidovudine. GM-CSF was administered as a daily (1.0 or 0.3 micrograms/kg) or every-other-day (0.3 micrograms/kg) subcutaneous dose over a 28-day period. Zidovudine was continued at the same daily dosage as was previously being administered. Of 11 patients, 7 (1.0 micrograms/kg, n = 5; 0.3 micrograms/kg, n = 2) had a pharmacologic response to GM-CSF with an increase to a mean absolute neutrophil count of 3,189 cells per mm3 at 4 weeks (P < 0.05). The peak concentration of GM-CSF in plasma ranged from 11.5 to 84.4 pg/ml, and the time to peak ranged from 1 to 3 h. No correlation between GM-CSF disposition and hematologic response was noted. A decreased plasma zidovudine-glucuronide/zidovudine ratio was noted after 1 week of GM-CSF, and an increase in the area under the plasma concentration-versus-time curve for zidovudine was found in three patients after 4 weeks. Low doses of GM-CSF can raise the granulocyte count in patients with zidovudine-induced neutropenia. The use of GM-CSF and zidovudine may represent a viable treatment option for persons with human immunodeficiency virus infection who develop neutropenia while receiving zidovudine but do not tolerate alternative nucleoside analogs. Further studies are needed to assess the complex interaction between these two agents.

Comparative pharmacokinetics of antiviral nucleoside analogues

The recent development of nucleoside analogues with antiviral activity has expanded the small but useful armamentarium for the treatment of certain viral diseases such as the human immunodeficiency virus, cytomegalovirus and others. Their intracellular site of action and need for sequential phosphorylation require that traditional pharmacokinetic parameters be used in conjunction with an understanding of intracellular metabolism when designing dosage regimens. This review summarises the available pharmacokinetic literature for zidovudine, didanosine, zalcitabine, aciclovir, ganciclovir, vidarabine and ribavirin. After oral administration, didanosine, aciclovir and ribavirin are < 50% bioavailable and ganciclovir is < 6% absorbed. In contrast, zidovudine and zalcitabine are > 60% bioavailable, although zidovudine undergoes considerable and variable first-pass hepatic glucuronidation while zalcitabine has no first-pass effect. Zidovudine, zalcitabine and didanosine are absorbed rapidly in the fasted state, with peak plasma concentrations exceeding their respective in vitro antiretroviral inhibitory concentrations. All reviewed agents except ribavirin have a relatively short plasma half-life (approximately 0.5 to 4h), with each agent demonstrating a different intracellular enzymatic activation scheme. For example, the rate-limiting step for formation of zidovudine triphosphate is the conversion of the monophosphate to the diphosphate, while didanosine is ultimately converted to dideoxyadenosine triphosphate which has the longest intracellular half-life (approximately 12 to 24h) among these agents. These drugs are not highly protein bound and they distribute into tissues with an apparent volume of distribution at steady-state ranging from 0.3 to 1.2 L/kg. They vary in the extent to which they enter cerebrospinal fluid, ranging from a low of < 25% for didanosine to a high of > 70% of a concurrent plasma concentration for ribavirin and vidarabine. These agents also vary with regard to degree of renal excretion of the parent drug, with the lowest noted for vidarabine (1 to 3%) and the highest for zalcitabine (approximately 75%) and ganciclovir (> 90%). With the increasing number of clinically useful nucleoside analogues, it is essential for the clinician to appreciate the subtle differences among these agents to ensure that optimal therapeutic outcomes may be attained with minimal toxicity.

Intravenous and oral zidovudine pharmacokinetics and coagulation effects in asymptomatic human immunodeficiency virus-infected hemophilia patients

Pharmacokinetic and coagulation studies were carried out over a 12-week period with 11 asymptomatic hemophilia patients with human immunodeficiency virus infection receiving zidovudine (ZDV). The patients received 300 mg every 4 h while awake (the accepted dose at the time of this study); consecutive 24-h intravenous (i.v.) and 12-h oral pharmacokinetic studies were conducted at weeks 1, 6, and 12. Coagulation studies were conducted at weeks 0, 4, 8, and 12. The numbers of units of factors VIII and IX and cryoprecipitate transfused during the 12-week periods before, during, and after ZDV treatment were recorded. Following i.v. and oral ZDV administration, the concentration in plasma declined rapidly over the first 4 h, and in some patients, ZDV was still detectable at 4 to 10 h. The i.v. total clearances (means +/- standard deviations) were 14.9 +/- 7.3, 11.2 +/- 3.7, and 15.1 +/- 4.7 ml/min/kg of body weight. The i.v. distribution volumes were 1.08 +/- 0.5, 1.0 +/- 0.4, and 1.65 +/- 1.4 liters/kg. The bioavailabilities were 0.54 +/- 0.22, 0.46 +/- 0.19, and 0.59 +/- 0.13 at weeks 1, 6, and 12, respectively. The pattern of ZDV-glucuronide (GZDV) disposition was similar to that of ZDV, and the peak plasma GZDV-to-ZDV ratio was higher after oral dosing, consistent with first-pass metabolism. In some individuals, up to 33% of an i.v. dose was excreted unchanged. At weeks 6 and 12, greater than 300 mg of total ZDV (GZDV plus ZDV) was recovered in the urine of some patients, suggesting tissue redistribution. Concentration in plasma after oral ZDV administration were variable, both within and between patients. The von Willebrand antigen level consistently decreased throughout the study but was not accompanied by a parallel change in ristocetin cofactor A activity, and no clinical adverse effects on coagulation were noted. This study demonstrates that ZDV can be used in hemophilia patients without worsening of their bleeding tendencies. The clinical significance of decreased ZDV clearance and the prolonged terminal elimination phase of ZDV will require further study with patients receiving chronic ZDV.

Trimetrexate for Pneumocystis carinii pneumonia in patients with AIDS

OBJECTIVE

The primary objective of this article is to introduce readers to the use of a new agent, trimetrexate (TMTX), in the treatment of Pneumocystis carinii pneumonia (PCP). The article also gives the readers an overview of PCP and discusses some of the controversies surrounding it. Pharmacokinetic data and clinical trials are reviewed, as well as adverse effects, drug interactions, and dosage guidelines.

DATA SOURCES

A MEDLINE search was used to identify pertinent literature, including reviews.

STUDY SELECTION

As both pharmacokinetic and clinical trials were few in number, all available trials were reviewed.

DATA EXTRACTION

Pharmacokinetic data from trials involving patients with AIDS was sparse; therefore, those involving oncology patients, including a pediatric population, were included. Although more trials need to be done in AIDS patients, the results from the oncologic trials give us a baseline from which to extrapolate. All clinical trials available at the time of publication were reviewed as were all of the preliminary results from three ongoing trials, which were made available through a personal communication.

DATA SYNTHESIS

TMTX has been found to be 1500 times more potent than trimethoprim as a dihydrofolate reductase inhibitor, and has the potential to provide an effective therapeutic option for PCP. TMTX is a lipid-soluble analog of methotrexate and is thus capable of greater penetration into Pneumocystis cells, which lack the folate membrane transport system necessary to take up classic folate structures like leucovorin and methotrexate, thereby negating any clinical effectiveness of methotrexate and allowing leucovorin to be used for host cell rescue. TMTX's pharmacokinetic parameters best fit a multicompartmental model with a terminal half-life of up to 12 hours. It is cleared both hepatically and renally with up to 41 percent excreted unchanged in the urine. Although TMTX's pharmacokinetic parameters are variable, the need for plasma concentration monitoring at present is unclear, as no dose-response relationship has been established.

Zidovudine update: 1990

Zidovudine (ZDV) is the only approved antiviral for the treatment of human immunodeficiency virus infection (HIV) in the U.S. Although newer antivirals have reached Phase II testing, ZDV is now the accepted therapy against which all other agents will be compared. Zidovudine 1500 mg/d was previously prescribed only to adult HIV-infected patients who had developed AIDS or AIDS-related complex (ARC). However, results obtained from recently completed studies indicate that a lower daily dose (500 mg) appears to be equivalent. In addition, ZDV therapy appears to be beneficial to asymptomatic HIV-infected patients with CD4+ counts less than 500/mm3. The toxicity profile of ZDV, previously obtained from patients receiving 1500 mg/d, consisted of either acute (e.g., fever, rash, headache) or chronic (e.g., anemia, neutropenia, myopathy) adverse effects. ZDV pharmacokinetics are variable within and between the different subpopulations of HIV-infected patients who have been studied. Bioavailability ranges from 50 to 70 percent, and values for half-life, total body clearance, and volume of distribution are 1-2 h, 20-40 mL/min/kg, and 1-2 L/kg, respectively. Drug interactions occur primarily between ZDV and other agents that undergo hepatic glucuronidation (e.g., probenecid, sulfamethoxazole) resulting in decreased ZDV clearance. ZDV is currently measured by HPLC, radioimmunoassay and FPIA; however, the role of therapeutic monitoring is currently under investigation. Studies of ZDV therapy in neonates, pediatric patients, patients with resistant isolates of HIV, and HIV-infected patients receiving combined treatment with other reverse transcriptase inhibitors or immunomodulators are ongoing.

2',3'-Dideoxyinosine in patients with AIDS or AIDS-related complex

Twenty-one patients with AIDS or AIDS-related complex (ARC) received 2',3'-dideoxyinosine (didanosine; ddI) intravenously and then orally (initial dosages of 0.4 mg/kg and 0.8 mg/kg every 12 hours, respectively) for 6-44 weeks in an escalating-dose study. The major dose-limiting effects were peripheral neuropathy (three patients) and pancreatitis (two patients), which were observed at dosages greater than or equal to 20 mg/(kg.d). Hyperuricemia occurred at greater than or equal to 30 mg/(kg.d). No hematologic toxicity developed except for possible sporadic thrombocytopenia (two patients). Significant decreases in serum levels of p24 antigen and increases in CD4+ and CD8+ lymphocytes were noted at 2, 6, and 10-20 weeks and over a wide range of dosages, including the lowest given. Most patients had an increased feeling of well-being and/or a weight gain of greater than or equal to 2 kg at 6 weeks. For this population, ddI has promise as a therapeutic agent, thus warranting further study of this agent in controlled clinical trials.

2',3'-dideoxyinosine (ddI) in patients with the acquired immunodeficiency syndrome or AIDS-related complex. A phase I trial

2',3'-Dideoxyinosine (ddI) is a purine analogue that after intracellular metabolic conversion suppresses the replication of the human immunodeficiency virus (HIV). We conducted a Phase I dose-escalation study of ddI in 17 patients with the acquired immunodeficiency syndrome (AIDS) and 20 patients with AIDS-related complex. The drug was administered twice daily over a dose range of 0.4 to 66 mg per kilogram of body weight per day for 2 to 44 weeks. The maximal tolerated oral dose of ddI was estimated to be 12 mg per kilogram per day. The major dose-limiting toxic effects were a painful peripheral neuropathy (in eight patients) and pancreatitis (in five). Asymptomatic elevations of the serum aminotransferase levels (in 13 patients) and the serum urate level (in 10) were also noted, but there was no dose-related hematologic toxicity. At the maximal tolerated dose, the peak plasma levels of ddI were 6.3 to 9.6 mumol per liter 0.6 to 1 hour after oral administration; the mean plasma half-life was 1.5 hours. The administration of ddI was associated with statistically significant decreases in serum level of p24 antigen and increases in the numbers of CD4 cells at 2, 6, 10, and 20 weeks. These changes were seen at all dose levels studied. Either a clinical improvement or a weight gain of greater than or equal to 2 kg was observed in 25 of 34 patients at six weeks. We conclude that ddI is a promising therapeutic agent in patients with AIDS or AIDS-related complex. Its efficacy is currently being evaluated in large-scale, controlled clinical trials.

Multiple-dose pharmacokinetics of oral zidovudine in hemophilia patients with human immunodeficiency virus infection

The disposition of zidovudine (ZDV) was examined during chronic oral dosing (300 mg every 4 h while awake) for 12 weeks in eight asymptomatic patients with hemophilia who were infected with the human immunodeficiency virus. Pharmacokinetic studies were conducted at the initiation of drug administration and after 6 and 12 weeks. Baseline liver function tests indicated normal values for bilirubin, albumin, and prothrombin time, while hepatic enzyme levels ranged from one to three times the normal levels. Initially, the mean peak ZDV concentration in plasma was 2,052 ng/ml with a range of 1,033 to 3,907 ng/ml, while during chronic dosing the peaks were 1,619 +/- 1,062 ng/ml and 1,711 +/- 786 ng/ml at weeks 6 and 12, respectively. ZDV concentrations at 4 h declined to 77 +/- 53 ng/ml, 110 +/- 43 ng/ml, and 101 +/- 49 ng/ml at weeks 1, 6, and 12, respectively. Initially, the plasma concentration-versus-time decay in three patients was linear, with a mean half-life (t1/2) of 1.3 +/- 0.5 h, while five patients had detectable concentrations in plasma after 4 h with an apparent delayed terminal-phase t1/2 of 4.8 +/- 2.8 h. At week 6 the prolonged elimination pattern was noted in all patients (terminal t1/2 = 4.1 +/- 2.0 h). No correlation between hepatic enzyme levels and t1/2 was noted. These findings suggest that ZDV may display a prolonged elimination phase during multiple dosing. Further studies utilizing a more sensitive assay may help to further define this later phase of ZDV elimination.

Methylprednisolone disposition in renal transplant recipients receiving triple-drug immunosuppression

Renal transplant patients commonly receive triple-drug immunosuppression with standardized doses of cyclosporine, azathioprine, and methylprednisolone. Although cyclosporine may decrease the clearance of oral prednisone, data are lacking for methylprednisolone, a glucocorticoid commonly prescribed via a standardized protocol for intravenous therapy and during periods of acute rejection. The disposition of methylprednisolone (doses: 10-60 mg/day) was examined in nine renal transplant patients during the post-transplant period (0.8-14 months). Plasma samples were collected over 24 hr and analyzed for methylprednisolone via HPLC. Pharmacokinetic parameters were determined by noncompartmental analysis. The mean total clearance of methylprednisolone was 379 ml/hr/kg (range 105-672) and the volume of distribution was 1.4 +/- 0.5 L/kg. The mean plasma half-life was 2.7 +/- 1.1 hr. When normalized to a 1 mg dose of methylprednisolone, the mean peak concentration at 1 hr was 10.0 +/- 3.5 ng/ml with an 8 hr concentration ranging from 0.3 to 5.5 ng/ml. An appreciable variability in methylprednisolone metabolism thus exists in renal transplant recipients receiving triple-drug immunosuppression. This may partially explain the variable response to steroid therapy during acute rejection episodes and chronic immunosuppression as well as the unpredictable occurrence of chronic steroid toxicity.

Comparison of cyclosporine assay methodology in the immediate postoperative period of renal transplantation

The method of measurement of cyclosporine concentrations in renal transplant recipients varies between centers and employs either high-performance liquid chromatography (HPLC) or radioimmunoassay (RIA). The merit of using HPLC for identifying the parent compound versus the RIA technique, which also measures certain cross-reactive metabolites that accumulate during renal impairment, is controversial. As a result of the lack of uniformity among centers, an abundance of complex literature that describes the disposition of this potent immunosuppressive agent, as well as a wide range of guidelines for therapeutic monitoring, has evolved. To examine the influence of assay methodology on the repeated determination of cyclosporine in the immediate postoperative period, a time when renal function is often unstable, eight renal transplant recipients were studied after i.v. and oral administration on up to four separate occasions. Whole-blood samples were analyzed by HPLC and RIA. Intravenous kinetic analysis yielded a mean total body clearance of 0.24 +/- 0.2 L/min (RIA) and 0.31 +/- 0.1 L/min (HPLC) (p greater than 0.05), the mean volume of distribution was 2.17 +/- 0.6 L/kg (RIA) and 2.75 +/- 1.2 L/kg (HPLC) (p greater than 0.05), and a mean half-life was 11.7 +/- 4.4 h (RIA) and 12.8 +/- 3.8 h (HPLC) (p greater than 0.05). The mean bioavailability was 0.36 +/- 0.23 (RIA) and 0.28 +/- 0.15 (HPLC) (p greater than 0.05). Regression of the 12-h cyclosporine (RIA versus HPLC) concentration yielded a line described by the following equation: RIA = 72 + 1.6 (HPLC).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of orally administered zidovudine among patients with hemophilia and asymptomatic human immunodeficiency virus (HIV) infection

Zidovudine (formerly azidothymidine, AZT) is used to treat certain patients infected with the human immunodeficiency virus (HIV). However, the clinical use of zidovudine (ZDV) in hemophilia patients may be complicated by the high incidence of chronic hepatitis in this patient population. To examine the pharmacokinetics of ZDV eight asymptomatic HIV-infected hemophilia patients received a single oral dose (300 mg). ZDV and its glucuronide metabolite (GZDV) were measured in serum by HPLC. ZDV was rapidly absorbed with a wide range of peak serum concentrations (2052 +/- 970 ng/ml) at 0.5 h. Peak GZDV serum concentrations were 4751 +/- 2269 ng/ml at 1 h. Both ZDV and GZDV declined in a biexponential manner over 4 h. After 4 h, the ZDV serum concentration decay in three patients continued a log-linear decline, while five patients demonstrated a tri-exponential curve which had a mean terminal elimination half-life of 4.8 +/- 2.8 h. No relationship between ZDV or GZDV kinetics and the degree of hepatic enzyme elevation was observed. Although a therapeutic window for ZDV has yet to be described, the wide range of serum concentrations that result from a standard dose suggests that clinical monitoring of ZDV levels may be of value in certain patients. In addition, the prolonged elimination half-life of ZDV in the present study may provide a rationale for less frequent dosing in certain patients.

Pharmacokinetics and clinical tolerance of intravenous and oral cyclosporine in the immediate postoperative period

The clinical tolerance and pharmacokinetics of cyclosporine during a prolonged intermittent intravenous infusion (3.5 mg/kg/day three times) followed by an 8 mg/kg daily oral dose was evaluated in eight renal transplant recipients in the immediate postoperative period. Cyclosporine was analyzed from whole blood samples by HPLC. Despite peak drug concentrations of 1463 +/- 754 ng/ml during the infusion period, no adverse pulmonary effects were noted; renal function, urine output, and mean arterial pressure also appeared to have been unaffected. The mean trough cyclosporine concentration was 141 +/- 50 ng/ml; however, two patients had trough values below sensitivity. Kinetic analysis after the third dose of intravenous cyclosporine revealed a mean total body clearance of 0.31 +/- 0.1 L/min and a volume of distribution of 2.88 +/- 1.1 L/kg, whereas the elimination half-life was 12.8 +/- 3.8 hours and the mean residence time was 9.5 +/- 5.1 hours. After conversion to oral therapy the bioavailability ranged from 0.11 to 0.47, with a mean value of 0.27. Subsequently there was an unpredictable pattern of bioavailability within patients, with mean values of 0.27 +/- 0.13 and 0.30 +/- 0.25 during the second and third oral study periods, respectively. These data suggest that despite adjusting the intravenous cyclosporine dosage to account for acute changes in patient body weight, variable kinetics may result in subtherapeutic trough values, even when cyclosporine is administered by prolonged infusion. The clinical implications of fluctuating cyclosporine bioavailability and a potential alternative approach to dosing are discussed.

Vancomycin-associated exfoliative dermatitis during continuous ambulatory peritoneal dialysis

Vancomycin is commonly prescribed to patients undergoing continuous ambulatory peritoneal dialysis (CAPD) for catheter-related infections and acute episodes of peritonitis. Although adverse dermatological reactions have been reported secondary to the rapid intravenous infusion of vancomycin, the intraperitoneal route of administration has been used routinely during CAPD without these effects. This case report describes a CAPD patient with systemic lupus erythematosus who developed erythema multiforme that progressed to exfoliative dermatitis during intermittent intraperitoneal vancomycin therapy for a catheter-related exit-site/tunnel infection.

Absorption of intraperitoneal antibiotics

The treatment of peritonitis in patients undergoing continuous ambulatory peritoneal dialysis (CAPD) usually includes the repeated administration of intraperitoneal (ip) antibiotics. The initial segment of this study (15 noninfected CAPD patients) examined the ip administration of four structurally different agents that represent the common types of antibiotics prescribed for peritonitis: an aminoglycoside (tobramycin), a glycopeptide (vancomycin), a beta-lactam (cefamandole), and an oxa-beta-lactam (moxalactam). Subsequently, 16 CAPD patients with peritonitis received either vancomycin (30 mg/kg) or cefamandole (1 g) in two liters of dialysate over a six-hour dwell period. Vancomycin and cefamandole were absorbed more rapidly in patients with peritonitis as indicated by a more rapid decline in dialysate concentrations, and higher serum concentrations that occurred earlier than in the noninfected patients. Although a higher percentage of the ip dose of vancomycin and cefamandole was absorbed during peritonitis, peak serum concentrations at the end of the drug administration dwell period were not significantly different. Numerous factors influence the absorption of ip antibiotics, including the dialysate drug concentration, the dwell period, protein binding, distribution volume, and presence or absence of peritonitis.

Once weekly intraperitoneal therapy for gram-positive peritonitis

The pharmacokinetics and clinical outcome following a 30 mg/kg/2 L intraperitoneal (IP) dose of vancomycin, which was administered once a week for 3 weeks, was studied in ten continuous ambulatory peritoneal dialysis patients with peritonitis. Vancomycin was 91% absorbed following the first dose and rapidly achieved therapeutic serum concentrations, 19 +/- 8 mcg/mL at 1 hour and a peak of 37 +/- 8 mcg/mL at 6 hours. Vancomycin was eliminated slowly with a mean total clearance of 7 +/- 3 mL/min/70 kg and a distribution volume of 1.2 +/- 0.3 L/kg. The resultant mean serum t1/2 over the first week was 184 hours and the mean serum concentration at 168 hours was 10 +/- 4 mcg/mL. Based on the positive clinical outcome (100% cure) among patients with uncomplicated gram-positive peritonitis, the potential use of this alternative vancomycin dosing regimen is proposed.

Overestimation of vancomycin concentrations utilizing fluorescence polarization immunoassay in patients on peritoneal dialysis

During a study of vancomycin pharmacokinetics in patients undergoing continuous ambulatory peritoneal dialysis (CAPD), a discrepancy was noted when serum concentrations were determined by high performance liquid chromatography (HPLC) in comparison to a fluorescence polarization immunoassay (FPI) technique. Following three weekly intraperitoneal doses (30 mg/kg/2 L), peak serum concentrations (at the end of the 6-h dwell) by FPI were 42.1 +/- 9.1, 43.1 +/- 8.7, and 45.6 +/- 7.4 micrograms/ml. In comparison, the same samples when analyzed by HPLC yielded 36.3 +/- 9.4, 32.2 +/- 8.9, and 31.6 +/- 9.1 micrograms/ml, respectively. A subsequent in vitro study of vancomycin (40 micrograms/ml) in serum indicated a degradation half-life of 693 (FPI) compared with 210 (HPLC) h. These data suggest that vancomycin degradation products accumulate in CAPD patients and lead to an overestimation of vancomycin serum concentrations when measured by FPI.