Use of drug effect interaction modeling with Monte Carlo simulation to examine the impact of dosing interval on the projected antiviral activity of the combination of abacavir and amprenavir

The delineation of optimal regimens for combinations of agents is a difficult problem, in part because, to address it, one needs to (i) have effect relationships between the pathogen in question and the drugs in the combination, (ii) have knowledge of how the drugs interact (synergy, antagonism, and additivity), and (iii) address the issue of true between-patient variability in pharmacokinetics for the drugs in the population. We have developed an approach which employs a fully parametric assessment of drug interaction using the equation of W. R. Greco, G. Bravo, and J. C. Parsons (Pharmacol. Rev. 47:331-385, 1995) to generate an estimate of effects for the two drugs and have linked this approach to a population simulator, using Monte Carlo methods, which produce concentration-time profiles for the drugs in combination. This software automatically integrates the effect over a steady-state dosing interval and produces an estimate of the mean effect over a steady-state interval for each simulated subject. In this way, doses and schedules can be easily evaluated. This software allows for a rational choice of dose and schedule for evaluation in clinical trials. We evaluated different schedules of administration for the combination of the nucleoside analogue abacavir plus the human immunodeficiency virus type 1 protease inhibitor amprenavir. Amprenavir was simulated as either 800 mg every 8 h (q8h) or 1,200 mg q12h, each along with 300 mg q12h of abacavir. Both regimens produced excellent effects over the simulated population of 500 subjects, with average percentages of maximal effect (as determined from the in vitro assays) of 90.9%+/- 11.4% and 80.9%+/-18.6%, respectively. This difference is statistically significant (P<<0.001). In addition, 68.8 and 46.0% of the population had an average percentage of maximal effect which was greater than or equal to 90% for the two regimens. We can conclude that the combination of abacavir plus amprenavir is a potent combination when it is given on either schedule. However, the more fractionated schedule for the protease inhibitor produced significantly better effects in combination. Clinicians need to explicitly balance the improvement in antiviral effect seen with the more fractionated regimen against the loss of compliance attendant to the use of such a regimen. This approach may be helpful in the preclinical evaluation of multidrug anti-infective regimens.

Combination therapy with amprenavir, abacavir, and efavirenz in human immunodeficiency virus (HIV)-infected patients failing a protease-inhibitor regimen: pharmacokinetic drug interactions and antiviral activity

Patients with plasma viral RNA >50,000 copies/mL, despite a protease-inhibitor regimen, received abacavir, amprenavir, and efavirenz to assess efavirenz-amprenavir drug interactions and to evaluate safety and antiviral response. Patients first received amprenavir with abacavir and other nucleoside analogs. Amprenavir levels were measured before and after adding efavirenz. Patients then received a second protease inhibitor. There was evidence of genotypic and phenotypic resistance at study entry. No patient had study drugs discontinued because of toxicity. Efavirenz decreased the steady-state area under the curve, maximum plasma concentration, and minimum plasma concentration of amprenavir by 24%, 33%, and 43%, respectively. Three of 10 patients had >1.5 log10 viral response to abacavir and amprenavir. All 8 patients who added efavirenz had >0.5 log10 decline in viral load, and this response lasted >24 weeks for 3 of the patients. A combination regimen that included abacavir, amprenavir, and efavirenz was well tolerated and had sustained activity in some patients. Concomitant efavirenz therapy decreases amprenavir concentrations.

Safety and pharmacokinetics of abacavir (1592U89) following oral administration of escalating single doses in human immunodeficiency virus type 1-infected adults

Abacavir (1592U89) is a nucleoside analog reverse transcriptase inhibitor that has been demonstrated to have selective activity against human immunodeficiency virus (HIV) in vitro and favorable safety profiles in mice and monkeys. A phase I study was conducted to evaluate the safety and pharmacokinetics of abacavir following oral administration of single escalating doses (100, 300, 600, 900, and 1,200 mg) to HIV-infected adults. In this double-blind, placebo-controlled study, subjects with baseline CD4+ cell counts ranging from < 50 to 713 cells per mm3 (median, 315 cells per mm3) were randomly assigned to receive abacavir (n = 12) or placebo (n = 6). The bioavailability of the caplet formulation relative to that of the oral solution was also assessed with the 300-mg dose. Abacavir was well tolerated by all subjects; mild to moderate asthenia, abdominal pain, headache, diarrhea, and dyspepsia were the most frequently reported adverse events, and these were not dose related. No significant clinical or laboratory abnormalities were observed throughout the study. All doses resulted in mean abacavir concentrations in plasma that exceeded the mean 50% inhibitory concentration (IC50) for clinical HIV isolates in vitro (0.07 microgram/ml) for almost 3 h. Abacavir was rapidly absorbed following oral administration, with the time to the peak concentration in plasma occurring at 1.0 to 1.7 h postdosing. Mean maximum concentrations in plasma (Cmax) and the area under the plasma concentration-time curve from time zero to infinity (AUC0-infinity) increased slightly more than proportionally from 100 to 600 mg (from 0.6 to 4.7 micrograms/ml for Cmax; from 1.0 to 15.7 micrograms.h/ml for AUC0-infinity) but increased proportionally from 600 to 1,200 mg (from 4.7 to 9.6 micrograms/ml for Cmax; from 15.7 to 32.8 micrograms.h/ml for AUC0-infinity. The elimination of abacavir from plasma was rapid, with an apparent elimination half-life of 0.9 to 1.7 h. Abacavir was well absorbed, with a relative bioavailability of the caplet formulation of 96% versus that of an oral solution (drug substance in water). In conclusion, this study showed that abacavir is safe and is well tolerated by HIV-infected subjects and demonstrated predictable pharmacokinetic characteristics when it was administered as single oral doses ranging from 100 to 1,200 mg.

Antiretroviral effect and safety of abacavir alone and in combination with zidovudine in HIV-infected adults. Abacavir Phase 2 Clinical Team

OBJECTIVES

To evaluate, over 12 weeks, the antiretroviral activity and safety of abacavir, used alone and in combination with zidovudine (ZDV), as treatment for HIV-1-infected subjects who had limited or no antiretroviral treatment.

DESIGN

Seventy-nine HIV-1-infected subjects, with CD4 cell counts 200-500 x 10(6)/l and <12 weeks of previous treatment with ZDV were enrolled in a multicenter study. Subjects were randomly assigned to one of four cohorts receiving abacavir monotherapy for the first 4 weeks (200, 400, or 600 mg every 8 h daily, or 300 mg every 12 h daily) and, thereafter, combination therapy of abacavir with 600 mg ZDV or ZDV placebo, administered in a double-blind manner for an additional 8 weeks.

METHODS

Antiretroviral activity was assessed by measuring changes in plasma HIV-1 RNA levels and CD4+ cell counts. Safety was assessed by monitoring clinical adverse events and laboratory abnormalities during the 12-week period and for 4 weeks post-treatment.

RESULTS

Treatment with abacavir, alone or in combination with ZDV, produced marked decreases in plasma HIV-1 RNA loads and increases in CD4+ cell counts in all groups. At week 4, median plasma HIV-1 RNA loads decreased by 1.11-1.77 log10 copies/ml and median CD4+ cell counts increased by 63-111 x 10(6)/l in all groups. At week 12, median HIV-1 RNA loads decreased by 1.02-2.24 log10 copies/ml (abacavir monotherapy) and by 1.81-2.01 log10 copies/ml (abacavir-ZDV); median CD4+ cell counts increased by 79-195 x 10(6)/l (abacavir monotherapy) and by 93-142 x 10(6)/l (abacavir-ZDV). At week 12, the percentage of subjects who had plasma HIV-1 RNA levels below 400 and 40 copies/ml were 28 and 11%, respectively (abacavir monotherapy) and 69 and 22%, respectively (abacavir-ZDV). Eight subjects (10%) discontinued the study prematurely because of adverse events; nausea (n = 4) and hypersensitivity (n = 3) were the most common reasons for withdrawal. There were no deaths among the study subjects.

CONCLUSIONS

In HIV-infected subjects who have received little or no prior antiretroviral therapy, treatment with abacavir alone or in combination with ZDV is well tolerated and resulted in sustained improvements in key immunologic and virologic efficacy parameters through 12 weeks.

Effects of aerosolized synthetic surfactant, atovaquone, and the combination of these on murine Pneumocystis carinii pneumonia

An immunosuppressed rat model was used to determine the pharmacokinetics of aerosolized atovaquone (administered with and without a synthetic surfactant) and to evaluate the efficacy of inhaled atovaquone in the prevention and treatment of Pneumocystis carinii pneumonia (PCP). After a single dose by aerosol, mean peak concentrations of atovaquone averaged 52 microg/mL in plasma and 31 microg/g in lungs of rats infected with P. carinii. When atovaquone was combined with surfactant, mean peak concentrations of 94 microg/mL in plasma and 51 microg/g in lung were achieved. Aerosolized synthetic surfactant alone significantly increased survival of rats with PCP and, when combined with atovaquone, increased plasma and lung concentrations of the drug and eradication of the organism.

Clinical experience with atovaquone on a treatment investigational new drug protocol for Pneumocystis carinii pneumonia

The clinical experience of human immunodeficiency virus (HIV) + patients treated with oral atovaquone for acute Pneumocytstis carinii pneumonia (PCP) under a Treatment Investigational New Drug (IND) protocol (mild or moderate PCP) and an Open-Label Study protocol (severe PCP) was evaluated. A total of 940 patients intolerant of or unresponsive to trimethoprimsulfamethoxazole were enrolled from private practices, clinics, and institutional HIV treatment centers in the United States. Demographics data and the history and severity of PCP were collected at enrollment. The number of therapy days, adverse experiences, clinical response to therapy, and mortality were collected at day 21. Reporting of serious, unexpected adverse experiences attributable to therapy was required. Of the 760 (96%) patients with mild to moderate disease for whom follow-up observation was complete, 591 (78%) responded clinically to treatment, 177 patients (23%) discontinued treatment prematurely, and 50 patients (7%) died. Of the 140 patients (95%) with severe PCP with follow-up data, 79 (56%) responded to treatment, 45 (32%) discontinued treatment early, and 53 patients (38%) died. Adverse events that resulted in temporary or permanent discontinuation of therapy included diarrhea, vomiting, elevated liver enzyme levels, nausea, and fever. No serious unexpected adverse events attributable to the drug were reported. The treatment IND mechanism enabled a large number of patients with acute PCP to be treated with this experimental therapy while the drug was under regulatory view.

A preliminary evaluation of 566C80 for the treatment of Pneumocystis pneumonia in patients with the acquired immunodeficiency syndrome

BACKGROUND

The drug 566C80 is an investigational hydroxynaphthoquinone that is active against Pneumocystis carinii in vitro and in animal models. Initial studies in humans indicate that 566C80 is safe and has adequate bioavailability after oral administration.

METHODS

We conducted an open-label trial of 566C80 in 34 adults with the acquired immunodeficiency syndrome (AIDS) and untreated pneumocystis pneumonia. All the patients had a partial pressure of arterial oxygen of at least 60 mm Hg while breathing room air. They were enrolled sequentially in three cohorts taking 566C80 at different dosages, all administered orally: 750 mg three times daily for 5 days, then twice daily for 16 days; 750 mg three times daily for 21 days; and 750 mg four times daily for 21 days.

RESULTS

All 34 patients survived, and 27 (79 percent) were successfully treated with 566C80 alone. The mean partial pressure of oxygen in 33 patients was 78 mm Hg at entry and 93 mm Hg after the course of 566C80 (P less than 0.001). In five patients (15 percent) the drug was discontinued because of lack of response. In four patients (12 percent), the drug was discontinued because of toxicity (fever and rash in two patients each). In two of these, treatment was considered to have succeeded because 566C80 was not discontinued because of toxicity until after day 14. Five of the successfully treated patients had rashes that resolved despite continued therapy. In nine patients, serum alanine aminotransferase levels rose above 100 U per liter. During the first three months after the completion of therapy, pneumocystis pneumonia recurred in 4 of the 27 successfully treated patients, and another 3 patients had recurrences between month 3 and month 6 of follow-up. The mean (+/- SEM) steady-state plasma levels of 566C80 were similar in the three cohorts: 16.3 +/- 2.10, 20.4 +/- 2.48, and 18.9 +/- 3.08 micrograms per milliliter in the patients taking the drug twice daily, three times daily, and four times daily, respectively.

CONCLUSIONS

From these preliminary data, the investigational compound 566C80 appears to be a safe, effective, and well-tolerated therapy for P. carinii pneumonia of mild-to-moderate severity in patients with AIDS.