a Department of Pharmacology and b Department of Infectious Diseases, IRCCS Policlinico S. Matteo, P. le Golgi 2, 27100 Pavia; c Department of Infectious Diseases, Istituti Ospitalieri, Cremona; d Department of Infectious Diseases, Ospedale S. Maria Annunziata, Firenze, Italy
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Abstract |
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Introduction |
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The most recently commercialized protease inhibitor, nelfinavir, has good inhibitory activity against HIV-1 and a favourable safety profile and has been shown to produce a large reduction in plasma HIV RNA concentrations when given in combination therapy with stavudine.7,8 In this study we have determined the pharmacokinetic profile of nelfinavir in combination therapy with efavirenz and stavudine in a cohort of heavily pretreated patients.
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Materials and methods |
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Eighteen HIV-1-infected patients in steady-state treatment with the combination of nelfinavir (750 mg every 8 h), efavirenz (600 mg od) and stavudine (3040 mg bd, according to bodyweight) were considered for pharmacokinetic evaluation. They were part of a broad clinical study to evaluate the safety and activity of this rescue regimen. All patients had reported intolerance, toxicity or disease progression when treated with protease inhibitor-based antiretroviral therapy. They were naive to any drug belonging to the non-nucleoside reverse transcriptase inhibitor class and had never been exposed to nelfinavir.
Patients who received the study drug for more than 4 consecutive weeks during the first 20 weeks of the study were eligible for pharmacokinetic sampling. Patients with concurrent use of medications known to be inhibitors or inducers of cytochrome P450 3A were excluded from this study.
Before they entered the pharmacokinetic study, the patients' medical histories were recorded and a physical examination and haematological, biochemical and virological tests were performed for each patient. Approval for the study was obtained from the local Ethics Committee and each patient provided written informed consent.
Blood samples were obtained during the dosing interval at the following times: 0 (before nelfinavir administration), 1, 2, 3, 4, 6 and 8 h after administration. Plasma samples were separated by centrifugation at 2500g for 5 min as soon as possible after the sampling time (not >1 h), inactivated in a bath at 56°C for 45 min and then frozen at 20°C until analysis.
Pharmacokinetic and statistical analysis
Plasma samples were analysed for nelfinavir concentrations with a validated HPLC assay with ultraviolet detection.9 Standard curves for the analytical method covered the range 0.05010.0 mg/L and linearity was evaluated using least squares regression analysis to plot the peak height ratio of nelfinavir to internal standard against nelfinavir concentration. The inter-assay and intra-assay coefficients of variation (CV%) of precision of the quality control samples (0.157.5 mg/L) ranged from 2.8 to 14.2%.
The effect of inactivation at 56°C on nelfinavir concentrations was investigated on three separate occasions with blank plasma samples spiked with different concentrations. Each sample was divided into two parts: the first heated at 56°C for 45 min, the second kept at room temperature. Both parts were then assayed by HPLC. The results obtained were not statistically different.
Nelfinavir concentrationtime data were analysed by compartmental and non-compartmental techniques. The statistical pharmacokinetic program P-Pharm Version 3 (Simed, Creteil, France)10 was used to determine the pharmacokinetic parameters of nelfinavir. P-Pharm uses a maximum a posteriori probability (MAP) Bayesian fitting procedure to combine prior knowledge with available information to estimate the individual parameters. The program also uses approximate statistical tests to evaluate the distribution properties of the differences between expected and observed data. Details of the mathematical presentation of the algorithm employed have been described previously.11
The pharmacokinetic parameters of nelfinavir were determined according to a one-compartment model. The following pharmacokinetic parameters were computed: Ka (absorption rate constant), Vd/F (distribution volume) and CL/F (total body clearance), where F is the fraction of the absorbed dose (assumed as equal to 1). The area under the plasma concentrationtime curve (AUC) during the dosing interval was calculated from DoseF/CL, and the average plasma drug concentration during the dosing interval at steady state (Css,av) was obtained from AUC/
. The trough plasma concentration (Css,min) was obtained from direct observation of data.
A non-compartmental pharmacokinetic analysis of nelfinavir plasma data was also carried out using the Pharm-NCA computer program (Simed).
The linear correlation coefficient (R) between the AUC08h of nelfinavir and the plasma concentration (C') was assessed at a specified time after drug administration in order to identify a sampling time that would provide the best estimate of total body exposure to the drug.
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Results |
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The results obtained from non-compartmental analysis of pharmacokinetic data gave similar results: Css,av = 2.13 ± 1.25 mg/L; AUC08h = 17.03 ± 10.03 mgh/L. The terminal half-life was 3.7 ± 1.3 h. The mean (± S.D.) trough plasma concentration of nelfinavir was 1.58 ± 1.00 mg/L (range 0.214.0 mg/L).
Examination of the relationship between the AUC08h of nelfinavir and C' at a time t' (0, 1, 2, 3, 4, 6 and 8 h) after the morning drug dose (Figure 2) showed that the best correlation occurred at t' = 6 h, expressed by the equation:
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Trough plasma concentrations of nelfinavir resulted in less accurate predictability for nelfinavir body systemic exposure:
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Discussion |
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Nelfinavir is extensively bound by human plasma protein (c. 98%). For nelfinavir a 0.4 mg/L plasma concentration is the stated minimum effective concentration (MEC) predicted on the basis of IC95 against several laboratory strains of HIV-1 and nelfinavir protein binding in vivo (J. Montaner, personal communication, 1997). In healthy volunteers a dosing regimen of 750 mg tds of nelfinavir leads to a mean Css,av of 2.5 ± 0.65 mg/L in plasma.19
In a preliminary evaluation of nelfinavir pharmacokinetics in HIV-positive patients,8 performed on day 28 of nelfinavir monotherapy (750 mg tds), the mean Css,av was 2.0 ± 0.9 mg/L and the mean Css,min was 1.3 ± 0.7 mg/L.
In seven patients with advanced HIV disease, receiving two nucleoside analogues in addition to nelfinavir 750 mg tds, Merry et al.20 reported a Css,av (geometric mean) of 2.9 mg/L (range 1.76.1 mg/L) and Cmin > 0.4 mg/L after 3 days of nelfinavir therapy. With the same dosing regimen and after 7 days of drug therapy, Skowron et al.21 in 23 HIV-positive patients achieved a mean (± S.D.) Css,av of 2.5 ± 0.93 mg/L.
In our patients, receiving one nucleoside analogue and efavirenz in addition to nelfinavir, the Css,av (mean 2.22 ± 1.25 mg/L) during an 8 h dosing interval ranged from 0.45 to 4.5 mg/L. Trough plasma concentrations (mean 1.58 ± 1.00 mg/L) ranged from 0.21 to 4.0 mg/L.
In healthy volunteers the coadministration of efavirenz (400600 mg od) was seen to result in a increase of c. 1520% in total body exposure to nelfinavir, with a corresponding decrease (37%) in the AUC of its hydroxylated active metabolite, AG 1402. From data reported by Fiske et al.,22 it appears that efavirenz and/or its metabolites inhibit one or more of the cytochrome P450 isoforms responsible for nelfinavir metabolism. In the absence of a control group receiving nelfinavir alone and because of major pharmacokinetic variability between individuals, we cannot detail this phenomenon in our patients. However, comparing the pharmacokinetic parameters of our patients with those reported for patients receiving nelfinavir alone or in combination with nucleoside analogues, we find a substantial overlap with nelfinavir concentrations achieved without efavirenz. This observation appears to confirm, in HIV-infected patients, that efavirenz combination does not require an a priori dose adjustment of nelfinavir, which was suggested by studies in healthy volunteers.
A nine-fold range in CL/F values of nelfinavir explains most of the high variability in the drug plasma concentrations observed between our patients. The nelfinavir plasma concentrations we found during the 8 h dosing interval did not remain above the suggested MEC (0.4 mg/L) in all patients; 4/18 patients (22%) showed plasma concentrations of nelfinavir that were less than the MEC.
To ensure that satisfactory plasma concentrations are achieved in all patients, a strategy of adjusting nelfinavir dosage based on measurements of total body systemic exposure should be further evaluated, with or without inclusion of efavirenz in the regimen.
Pre-dose trough concentrations are generally used to indicate drug exposure. However, since there is little information on the relationship between nelfinavir trough concentrations and the AUC, we tried to characterize the relationship between the AUC over 8 h and single point measurements of nelfinavir. A single drug determination 6 h (C6h) after nelfinavir administration best correlated with AUC08h compared with the other single point determinations. Nelfinavir exposure can be accurately calculated as follows:
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Trough plasma concentrations of nelfinavir make less accurate predictors of the AUC and can predict 83% of drug exposure compared with 97% when a 6 h single point is used. Alternatively, a 2 h abbreviated AUC calculated by the nelfinavir plasma concentration of the morning pre-dose sample and of 1 h and 2 h post-dose samples provided an estimate of the full 08 h (or interdose) AUC as good as the 6 h single point (Figure 2):
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An abbreviated profile with samples collected over a shorter time period might be preferred as a more practical approach and can also be applied in the outpatient setting for drug monitoring. However, it should be noted that none of the patients in this study had abnormal liver function.
Although both these approaches seem to offer consistent advantages over the use of trough plasma concentrations and represent an effective strategy for drug interaction studies and for inter/intra-patient variability monitoring (e.g. assessing adherence), there are no definitive results from studies evaluating therapeutic drug monitoring of nelfinavir for routine care. Achievement of adequate plasma concentrations of nelfinavir (as for other PIs) is clearly one factor in successful therapy, but further investigations on pharmacokineticpharmacodynamic correlations are needed to identify appropriate concentrations for optimal therapeutic efficacy and for safety.
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Notes |
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References |
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Received 8 April 1999; returned 23 September 1999; revised 25 October 1999; accepted 5 November 1999