Interaction between saquinavir soft-gel and rifabutin in patients infected with HIV.

AIMS: To evaluate the potential pharmacokinetic interaction between the HIV protease inhibitor saquinavir and rifabutin. METHODS: Fourteen HIV-infected patients provided full steady-state pharmacokinetic profiles following administration of rifabutin alone (300 mg once daily) or saquinavir soft-gel formulation (1200 mg three times daily) plus rifabutin (300 mg once daily) in this open label, partially randomized study. RESULTS: Coadministration of saquinavir and rifabutin resulted in a reduction in saquinavir AUC(0,8 h) and C(max)(0,8 h) of 47% (95% CI 30, 60%) and 39% (95% CI 11, 59%), respectively. Rifabutin AUC(0,24 h) and C(max)(0,24 h) was increased by an average of 44% (95% CI 17, 78%) and 45% (95% CI 14, 85%), respectively. Saquinavir in combination with rifabutin was well tolerated. Gastrointestinal intolerance and asymptomatic increases in liver enzymes were the only adverse events of note. CONCLUSIONS: Administration of rifabutin with saquinavir may decrease the efficacy of this HIV protease inhibitor.

The effect of ritonavir on saquinavir plasma concentration is independent of ritonavir dosage: combined analysis of pharmacokinetic data from 97 subjects.

OBJECTIVE: To determine the correlation between ritonavir (RTV) dose and the degree of enhancement of saquinavir (SQV) exposure. METHODS: Combined analysis of pharmacokinetic data at steady state obtained from two open-label, randomized, parallel-group, multiple-dose, single-centre studies involving healthy volunteers. Plasma samples for SQV assay were obtained from 97 healthy subjects following multiple dosing of a range of SQV (400-1800 mg) plus RTV (100-400 mg) dosages for 13-14 days. The pharmacokinetics of SQV were derived by model-independent, noncompartmental methods. Data were analysed by multivariate regression of log transformed Cmin and Cmax (geometric means) of SQV dosage as the dependent variable and independent variables of SQV and RTV dosage. Ritonavir was fitted as both a continuous and a categorical variable. RESULTS: There is a strong effect of any dose of RTV on Cmax and Cmin of SQV (P < 0.0001 for both parameters), but no greater effect of higher vs. lower RTV dosages on either parameter (Cmax: P=0.4373; Cmin: P=0.3393). Higher SQV dosage correlates linearly with higher Cmax (P=0.0093) and Cmin (P=0.0010), but the effects of increasing SQV dosages are less than with the addition of any RTV dose. CONCLUSIONS: RTV enhances SQV concentrations to increase Cmax and Cmin. This effect is similar for RTV dosages of 100-400 mg twice daily. Based on this concept of 'mini-dose' RTV, once-daily dosing of 1600 mg SQV/100 mg RTV and twice-daily 1000 mg SQV/100 mg RTV are currently being evaluated in clinical trials.

Saquinavir and ritonavir pharmacokinetics following combined ritonavir and saquinavir (soft gelatin capsules) administration.

AIMS: To investigate the influence of combined ritonavir (RTV) and saquinavir (soft-gelatin capsule formulation; SQV) on systemic exposure to SQV with a view to optimizing the dosing regimen of combined RTV and SQV antiretroviral therapy. METHODS: In this open labelled, randomized, parallel group study, SQV and RTV were administered twice daily for 14 days to groups of eight healthy subjects. The two antiretrovirals were either administered alone (800 mg SQV, regimen A, and 400 mg RTV, B) or in combination at various dose levels (RTV : SQV: 400 : 400 mg, C; 300 : 600 mg, D; 200 : 800 mg, E; 300 : 800 mg, F; 400 : 800 mg, G; and 400 : 600 mg, H). Pharmacokinetic parameters of saquinavir and ritonavir were determined and adverse events, vital signs, and clinical laboratory variables recorded. RESULTS: RTV substantially increased the plasma concentration of saquinavir for all dose combinations, compared with SQV alone. Based on the primary statistical analysis there was an overall 17-, 22-, and 23-fold increase in saquinavir AUC(0,24 h) on day 14 with regimens E, F, and G, respectively (with confidence intervals of 10-30, 13-37, and 13-39). The lowest combination dose of RTV (200 : 800 mg; E) significantly increased the saquinavir AUC(0,24 h) from below 5 to 57 microg ml(-1) h, which was higher than the exposure obtained with the 400 : 400 mg twice daily regimen (i.e. 36 microg ml(-1) h). RTV also reduced intersubject variability in AUC(0,24 h) for saquinavir from 105% to 32-68%, and C(max)(0,24 h) from 124% to 30-49%. In contrast, SQV showed no clinically significant effect on the pharmacokinetics of ritonavir. The combination regimens were well tolerated, with the least number of adverse events recorded for the 200 : 800 mg (RTV : SQV) combination regimen. CONCLUSIONS: RTV significantly increases saquinavir exposure as a consequence of inhibiting SQV metabolism and possibly P-glycoprotein efflux. Pharmacokinetic and safety profiles obtained in the current study indicate that the use of a combination with a lower dose of RTV and a higher dose of SQV than the 400 : 400 mg combination frequently used in clinical practice should be further explored.

Combination therapy with saquinavir soft gelatin capsules in children with human immunodeficiency virus infection.

OBJECTIVES: To evaluate the pharmacokinetics, tolerance, safety and antiviral activity of the HIV protease inhibitor, saquinavir, formulated as soft gelatin capsules (SQV-SGC), given in combination with nucleoside antiretroviral agents (NRTIs) with or without nelfinavir in HIV-infected children. METHODS: This was an open label study of HIV-infected children ages 3 to 16 years, conducted in two parts. In Part 1 of the study 14 children were treated orally with SQV-SGC (initially given in three 33-mg/kg doses daily; dosage adjusted to 50 mg/kg three times daily based on initial pharmacokinetics) and two NRTIs. Addition of nelfinavir was permitted for children who did not achieve a predetermined steady state target plasma saquinavir exposure. In Part 2 a new group of 13 children received SQV-SGC (33 mg/kg three times daily) in combination with nelfinavir and one or two NRTIs. Pharmacokinetics were assessed after the first dose and 4 weeks into treatment (steady state). Patients were treated for 72 and 48 weeks in Parts 1 and 2, respectively. RESULTS: Most adverse events were mild; the most commonly reported were diarrhea, abdominal discomfort and headache. Two children were withdrawn from the study because of adverse events (one each of nausea and dysphagia) related to the study treatment. There were no deaths or serious adverse events attributed to the study medication. Steady state saquinavir area under the plasma concentration vs. time curves (AUC24) were 6,210 and 11,010 ng/h/ml for Parts 1 and 2, respectively. Compared with baseline measurements median changes in plasma HIV RNA concentrations were -2.12 log10 copies/ml [5 of 14 (36%) with HIV RNA <50 copies/ml) (Week 72)] and -2.58 log10 copies/ml [8 of 13 (62%) <50 copies/ml) (Week 48)] in Parts 1 and 2, respectively. The median changes in CD4+ lymphocyte count were +292 and +154 cells/microl for Parts 1 and 2, respectively. Genotypic resistance assays revealed a low frequency of saquinavir-associated resistance mutations after 48 weeks of therapy, with only 2 of 27 children having substitutions at positions 48V and/or 90M. CONCLUSIONS: Combination therapy with SQV-SGC was well-tolerated and safe in HIV-infected children, and antiviral activity was observed. Saquinavir plasma concentrations were lower than expected, particularly for Part 1 (SQV-SGC plus NRTIs), but addition of nelfinavir increased saquinavir exposures.

Measuring the effectiveness of antiretroviral agents.

Considerable progress has been made recently in developing effective antiretroviral combination therapy that can suppress viral replication and delay disease progression in individuals infected with HIV. A range of up to 15 approved antiretroviral agents is now available, which target two different viral enzymes, while several agents are in clinical development. The rapid development and approval of antiretroviral agents, driven by the urgency of clinical need as well as the complexity of possible combinations, has precluded the extensive comparative clinical testing of regimens, which is necessary to establish the relative efficacy of various different agents. The lack of an appropriate animal model for HIV disease also increases reliance on in vitro measures. Several different in vitro and in vivo parameters have been defined in an attempt to quantify the effectiveness of antiretroviral agents, most importantly the 50% inhibitory and effective concentrations (IC50 and EC50). However, the clinical relevance of these measures is uncertain. Additionally, considerable variation exists in the usage of the terms 'IC50' and 'EC50' in recent publications in the literature. These issues pose interpretation problems to clinicians seeking information on the relative clinical efficacy of the agents. In this brief review, we attempt to clarify the different measures available and their potential utility for clinical decision-making, focusing particularly on the example of HIV protease inhibitors. There are many different quantifiable parameters that give information regarding the effectiveness of an antiviral drug. These include: inhibition of the viral target enzyme (inhibition constant, Ki); selectivity for viral versus host enzymes; inhibition of viral replication in cell culture (IC50); ratio of efficacy to cytotoxicity in vitro (therapeutic index); inhibition of viral replication or symptoms in an appropriate animal model of the disease (EC50); and the effect on surrogate markers, such as viral load or CD4 cell count, after administration to humans (in vivo EC50). Each of these different parameters gives valid information about the properties of an antiretroviral agent, which can help to build up a picture of its potential clinical utility relative to other drugs. However, to gain meaningful results, it is important to apply this information intelligently, understanding the limitations of each parameter.

Safety and pharmacokinetics of once-daily regimens of soft-gel capsule saquinavir plus minidose ritonavir in human immunodeficiency virus-negative adults.

Human immunodeficiency virus type 1 (HIV-1) protease inhibitors have dramatically improved treatment options for HIV infection, but frequent dosing may impact adherence to highly active antiretroviral treatment regimens (HAART). Previous studies demonstrated that combined therapy with ritonavir and saquinavir allows a decrease in frequency of saquinavir dosing to twice daily. In this study, we evaluated the safety and pharmacokinetics of combining once-daily doses of the soft-gel capsule (SGC) formulation of saquinavir (saquinavir-SGC) and minidose ritonavir. Forty-four healthy HIV-negative volunteers were randomized into groups receiving once-daily doses of saquinavir-SGC (1,200 to 1,800 mg) plus ritonavir (100 to 200 mg) or a control group receiving only saquinavir-SGC (1,200 mg) three times daily. Saquinavir-SGC alone and saquinavir-SGC-ritonavir combinations were generally well tolerated, and there were no safety concerns. Addition of ritonavir (100 mg) to saquinavir-SGC (1,200 to 1,800 mg/day) increased the area under the concentration-time curve (AUC) for saquinavir severalfold, and the intersubject peak concentration in plasma and AUC variability were reduced compared to those achieved with saquinavir-SGC alone (3,600 mg/day), while trough saquinavir levels (24 h post-dose) were substantially higher than the 90% inhibitory concentration calculated from HIV-1 clinical isolates. Neither increasing the saquinavir-SGC dose to higher than 1,600 mg nor increasing ritonavir from 100 to 200 mg appeared to further enhance the AUC. These results suggest that an all once-daily HAART regimen, utilizing saquinavir-SGC plus a more tolerable low dose of ritonavir, may be feasible. Studies of once-daily saquinavir-SGC (1,600 mg) in combination with ritonavir (100 mg) in HIV-infected patients are underway.

Pharmacokinetic interactions between sildenafil and saquinavir/ritonavir.

AIMS: To investigate the effect of the antiretroviral protease inhibitors saquinavir (soft gelatin capsule) and ritonavir on the pharmacokinetic properties and tolerability of sildenafil and to investigate the effect of sildenafil on the steady-state pharmacokinetics of saquinavir and ritonavir. METHODS: Two independent, 8 day, open, randomized, placebo-controlled, parallel-group studies (containing a double-blind crossover phase) were conducted at Pfizer Clinical research units (Canterbury, UK. and Brussels, Belgium). Twenty-eight healthy male volunteers entered each study. In each study, volunteers were randomized (n = 14 per group) to receive sildenafil on day 1 followed by a 7-day treatment period (days 2-8) with saquinavir or placebo (Study I) or ritonavir or placebo (Study II). Sildenafil or placebo (Study I and Study II) was administered alternately on day 7 or day 8, depending on initial randomization. The effect of saquinavir and ritonavir on the pharmacokinetics of sildenafil and its primary circulating metabolite (UK-103, 320) and the effect of single-dose sildenafil on the steady-state pharmacokinetics of saquinavir (1200 mg three times daily) and ritonavir (500 mg twice daily) were determined. The safety and tolerability of sildenafil coadministered with saquinavir or ritonavir were also assessed. RESULTS: Both protease inhibitors significantly increased Cmax, AUC, tmax and t(1/2) values for both sildenafil and UK-103, 320. Ritonavir showed a significantly greater effect than saquinavir with increases in sildenafil AUC and Cmax of 11-fold (95% CI: 9.0, 12.0) and 3.9-fold (95% CI: 3.2, 4.9), respectively. This compared with increases of 3.1-fold (95% CI: 2.5, 4.0) and 2.4-fold (95% CI: 1.8, 3.3) for coadministration with saquinavir. In contrast, the steady-state pharmacokinetics of saquinavir and ritonavir were unaffected by sildenafil. The increases in systemic exposure to sildenafil and UK-103, 320 were not associated with an increased incidence of adverse events or clinically significant changes in blood pressure, heart rate or ECG parameters. CONCLUSIONS: These results indicate that both saquinavir and ritonavir modify the pharmacokinetics of sildenafil presumably through inhibition of CYP3A4. The more pronounced effect of ritonavir may be attributed to its additional potent inhibition of CYP2C9. No change in safety or tolerability was observed when sildenafil was coadministered with either protease inhibitor. However, given the extent of the interactions, a lower sildenafil starting dose (25 mg) should be considered for patients receiving saquinavir and it is recommended not to exceed a maximum single dose of 25 mg in a 48 h period for patients receiving ritonavir.

Relationships between exposure to saquinavir monotherapy and antiviral response in HIV-positive patients.

OBJECTIVE: The aim of this study was to confirm the most appropriate dosage of a new soft gelatin capsule (SGC) formulation of the HIV protease inhibitor saquinavir by investigating the relationships between systemic (plasma) exposure to saquinavir and plasma HIV RNA and CD4+ cell counts using empirical mathematical modelling. DESIGN AND SETTING: A randomised, non-blind, multicentre, dose-ranging 8-week study of monotherapy with 400, 800 or 1200 mg of saquinavir-SGC or 600 mg of the hard gelatin capsule (HGC) formulation, both administered 3 times daily, was carried out in protease inhibitor-naive, HIV-positive adults. Two surrogate markers of response, plasma HIV RNA level and CD4+ cell count, were fitted to 2 measures of systemic drug exposure, the area under the plasma concentration-time curve (AUC) and trough plasma concentration (Cmin), using 6 exposure-response models of progressively increasing complexity. Akaike and Schwarz model selection criteria were applied to determine the most effective pharmacokinetic-pharmacodynamic relationship. RESULTS: A total of 88 patients were randomised; pharmacokinetic and pharmacodynamic data were available for 84 patients. In terms of plasma HIV RNA, pharmacokinetic-pharmacodynamic relationships were best described by a 2-parameter maximum effect (Emax) model, which predicted a typical maximum reduction in viral load of 1.94 log10 copies/ml [coefficient of variation (CV) 12%], with a half-maximal antiviral response occurring at a Cmin of 50 micrograms/L (CV 40%). Saquinavir-SGC 1200 mg administered 3 times daily produced a median AUC to 24 hours (AUC24) of approximately 20,000 micrograms/L.h, corresponding to 85% of the maximum achievable antiviral effect as defined by the model. None of the models yielded a satisfactory fit for CD4+ cell count. CONCLUSION: Empirical mathematical modelling confirmed that, when administered 3 times daily, the optimum dose of saquinavir-SGC is 1200 mg, corresponding to 3600 mg/day.

All-trans-retinoic acid in maternal plasma and teratogenicity in rats and rabbits.

The teratogenicity of all-trans-retinoic acid, 13-cis-retinoic acid, and retinol was investigated in pregnant Wistar rats given a single oral dose on Day 10 of gestation. External malformations showed dose-dependent increases and the order of potency was all-trans-retinoic acid > retinol > 13-cis-retinoic acid. The metabolites in maternal plasma were determined following a single oral dose on Day 10 of gestation. Equipotent teratogenic doses of all-trans-retinoic acid and 13-cis-retinoic acid had similar plasma levels of all-trans-retinoic acid; however, retinol teratogenicity could not be accounted for by circulating all-trans-retinoic acid or its metabolites. The teratogenicity and maternal pharmacokinetics of all-trans-retinoic were compared in pregnant Wistar rats when given as a single dose (50 mg/kg) and as three equal doses (16.66 mg/kg) over 6 hr. Divided doses were of slightly greater potency than the single dose but the maximum observed concentration (Cmax) and area under the plasma concentration-time curve (AUC) values for the second and third doses were greatly attenuated compared with the first dose; in consequence, both the total AUC and Cmax were reduced compared with the single dose. The altered profile could not be explained by increased formation of all-trans-retinoic acid glucuronide or increased isomerisation to 13-cis-retinoic acid. The maternal plasma levels of all-trans-retinoic acid in pregnant rabbits were reduced by a dose given 24 hr earlier. These data show that all-trans-retinoic acid in maternal plasma is a poor indicator of fetal exposure to teratogenic risk.

The teratogenic metabolites of vitamin A in women following supplements and liver.

Ten healthy female volunteers were given 5 doses of retinol as the palmitate; 50 and 150 mg retinol as an oral supplement, 50 and 150 mg as fried calf liver (50 and 150 g) and 3, 9 or 30 mg by intra-muscular injection. Plasma concentrations of retinyl palmitate were higher after 50 mg retinol given as an oral supplement compared with 50 mg as liver; there was no significant difference between the 150 mg doses. Plasma concentrations of retinol showed only small increases. The peak plasma concentrations (Cmax) of all-transretinoic acid, the principal teratogenic metabolite of retinol, and the area under the concentration-time curve (AUC) were up to 20-times higher after supplements compared to the same dose as liver. Plasma concentrations of all-trans-4-oxo-retinoic acid, 13-cis-retinoic acid and 13-cis-4-oxo-retinoic acid showed smaller differences between supplements and liver. Intra-muscular administration of 30 mg retinol gave retinyl palmitate concentrations similar to those found after the oral doses but did not increase circulating concentrations of the acid metabolites. Based on the formation of all-trans-retinoic acid, liver and supplements are not of equivalent teratogenic potential. Advice to pregnant women on the consumption of liver based on the reported teratogenicity of vitamin A supplements should be reconsidered.

Blood pressure changes and sympathetic function in rats given cyclohexylamine by intravenous infusion.

Intravenous infusion of cyclohexylamine (30-120 mg/kg) caused a dose-dependent increase in blood pressure in urethane-anesthetized rats. The increase in blood pressure was inversely related to the duration of the infusion. The blood pressure returned to baseline values (+/- 5 mm Hg) within 60 min of the end of the infusion after doses of 30 and 60 mg/kg were administered over 20 and 40 min. The plasma concentrations of cyclohexylamine were related linearly to the administered dose and decreased only 27% between the end of the infusion and 60 min later. The concentration-effect relationship showed clockwise hysteresis, indicative of tachyphylaxis, as has been reported in humans (Eichelbaum et al., 1974). Administration of bolus doses of tyramine at the end of the infusion or 60 min later demonstrated the presence of an indirect sympathomimetic response, although this was attenuated to a greater extent by high doses and more rapid infusions of cyclohexylamine. An almost complete loss of response to tyramine was found only in animals given 120 mg/kg over 40 min. The presence of a tyramine response 60 min after the infusion of 60 mg/kg occurred when there was an essentially complete reversal of the hypertensive effect of cyclohexylamine. These data indicate that the hypertensive effect of the indirectly acting sympathomimetic amine cyclohexylamine occurs primarily during rapid increases in plasma concentrations. Tachyphylaxis develops rapidly after the cessation of the infusion which is probably due largely to reuptake of released noradrenaline at low doses and depletion of releasable noradrenaline at high doses.

Oxidative and non-oxidative metabolism of ethanol by the rabbit lung.

Pulmonary ethanol metabolism in the rabbit was investigated in vitro by examining oxidative (alcohol dehydrogenase) and non-oxidative (fatty acid ethyl ester synthase) pathways. Pulmonary alcohol dehydrogenase (ADH) activity was very low when measured at close to normal physiological conditions but increased at higher pH, although it was lower than that in the liver. Significant esterification of oleic acid was detected using rabbit lung homogenates, with the rate being intermediate to that of the other tissues examined. The results indicate that oxidative metabolism of ethanol (via ADH) by the rabbit lung is minimal compared to other tissues, but non-oxidative metabolism of ethanol leading to FAEE formation occurs at significant rates that may alter lipid metabolism or membrane composition.

The metabolism of cyclamate to cyclohexylamine and its cardiovascular consequences in human volunteers.

A group of 194 diabetic patients were given calcium cyclamate (1 g/day as cyclamic acid equivalents) for a period of 7 days. Blood and urine samples were collected to determine the formation of cyclohexylamine, which is an indirectly acting sympathomimetic amine. Blood pressure and heart rate were recorded before and after treatment. Urine samples were collected each day and analyzed for cyclamate (to check compliance) and cyclohexylamine (to monitor the development of metabolizing activity). After 7 days intake most individuals (78%) did not excrete significant amounts of cyclohexylamine (less than 0.1% of the daily dose of cyclamate) but a small number (8; 4% of the group) excreted more than 20% of the daily dose as cyclohexylamine in the urine. Similar interindividual variations were found in the plasma concentrations of cyclohexylamine after 7 days intake of cyclamate, with 8 individuals having concentrations of 300-1942 ng/ml. The changes in cardiovascular parameters in these 8 subjects between pre- and postdosing were similar to those found in 150 subjects with plasma cyclohexylamine concentrations less than 10 ng/ml. Twenty of the subjects were restudied after receiving calcium cyclamate for 2 weeks at a daily dose equivalent to 2 g of cyclamic acid (0.66 g tds). Plasma concentrations of cyclohexylamine, heart rate, and blood pressure were measured every 30 min for a period of 8 hr (one dose interval) after the final dose. Twelve patients had plasma concentrations of cyclohexylamine greater than 10 ng/ml (89-2043 ng/ml) at the start of the dose-interval investigations. There were no transient increases or decreases in plasma concentrations of cyclohexylamine which might have resulted in a transient change in blood pressure or heart rate. These data indicate that the metabolism of cyclamate (2 g/day) to cyclohexylamine would not affect blood pressure or heart rate even in individuals with high metabolizing ability.