Nelfinavir

HIV protease is an enzyme required for the proteolytic cleavage of the viral polyprotein precursors to the individual proteins found in infectious HIV. The cleavage of these viral polyproteins is essential for the maturation of infectious virus. Nelfinavir reversibly binds to the active site of HIV protease and prevents cleavage of the polyproteins resulting in the formation of immature non-infectious viral particles.

Antiviral activity in vitro

The antiviral activity of nelfinavir in vitro has been demonstrated in both HIV acute and chronic infections in lymphoblastoid cell lines, peripheral blood lymphocytes and monocytes/macrophages. Nelfinavir was found to be active against a broad range of laboratory strains and clinical isolates of HIV-1 and the HIV-2 strain ROD. The EC₉₅ (95% effective concentration) of nelfinavir ranged from 7 to 111 nM (mean of 58 nM). Nelfinavir demonstrated additive to synergistic effects against HIV in combination with reverse transcriptase inhibitors zidovudine (ZDV), lamivudine (3TC), didanosine (ddI), zalcitabine (ddC) and stavudine (d4T) without enhanced cytotoxicity.

Resistance

Viral escape from nelfinavir can occure via viral protease mutations at amino acid positions 30, 88 and 90.

In vitro

HIV isolates with reduced susceptibility to nelfinavir have been selected in vitro. HIV isolates from selected patients treated with nelfinavir alone or in combination with reverse transcriptase inhibitors were monitored for phenotypic (n=19) and genotypic (n=195, 157 of which were assessable) changes in clinical trials over a period of 2 to 82 weeks. One or more viral protease mutations at amino acid positions 30, 35, 36, 46, 71, 77 and 88 were detected in >10% of patients with assessable isolates. Of 19 patients for whom both phenotypic and genotypic analyses were performed on clinical isolates, 9 patients isolates showed reduced susceptibility (5- to 93-fold) to nelfinavir in vitro. Isolates from all 9 patients possessed one or more mutations in the viral protease gene. Amino acid position 30 appeared to be the most frequent mutation site.

Cross resistance in vitro

HIV isolates obtained from 5 patients during nelfinavir therapy showed a 5- to 93-fold decrease in nelfinavir susceptibility in vitro when compared to matched baseline isolates but did not demonstrate a concordant decrease in susceptibility to indinavir, ritonavir, saquinavir or amprenavir in vitro. Conversely, following ritonavir therapy, 6 of 7 clinical isolates with decreased ritonavir susceptibility (8- to 113-fold) in vitro compared to baseline also exhibited decreased susceptibility to nelfinavir in vitro (5- to 40 fold). An HIV isolate obtained from a patient receiving saquinavir therapy showed decreased susceptibility to saquinavir (7- fold) but did not demonstrate a concordant decrease in susceptibility to nelfinavir. Cross-resistance between nelfinavir and reverse transcriptase inhibitors is unlikely because different enzyme targets are involved. Clinical isolates (n=5) with decreased susceptibility to zidovudine, lamivudine, or nevirapine remain fully susceptible to nelfinavir in vitro.

In vivo

The overall incidence of the D30N mutation in the viral protease of assessable isolates (n=157) from patients receiving nelfinavir monotherapy or nelfinavir in combination with zidovudine and lamivudine or stavudine was 54.8%. The overall incidence of other mutations associated with primary PI resistance was 9.6% for the L90M substitution where as substitutions at 48, 82 and 84 were not observed.

The pharmacokinetic properties of nelfinavir have been evaluated in healthy volunteers and HIV-infected patients. No substantial differences have been observed between healthy volunteers and HIV-infected patients.

Absorption

After single or multiple oral doses of 500 to 750 mg (two to three 250 mg tablets) with food, peak nelfinavir plasma concentrations were typically achieved in 2 to 4 hours.

After multiple dosing with 750 mg every 8 hours for 28 days (steady-state), peak plasma concentrations (C_max) averaged 3-4 μg/ml and plasma concentrations prior to the next dose (trough) were 1-3 μg/ml. A greater than dose-proportional increase in nelfinavir plasma concentrations was observed after single doses; however, this was not observed after multiple dosing.

A pharmacokinetic study in HIV-positive patients compared multiple doses of 1250 mg twice daily (BID) with multiple doses of 750 mg three times daily (TID) for 28 days. Patients receiving nelfinavir BID (n=10) achieved nelfinavir C_max of 4.0 ± 0.8 μg/ml and morning and evening trough concentrations of 2.2 ± 1.3 μg/ml and 0.7 ± 0.4 μg/ml, respectively. Patients receiving nelfinavir TID (n=11) achieved nelfinavir peak plasma concentrations (C_max) of 3.0 ± 1.6 μg/ml and morning and evening trough concentrations of 1.4 ± 0.6 μg/ml and 1.0 ± 0.5 μg/ml, respectively. The difference between morning and afternoon or evening trough concentrations for the TID and BID regimens was also observed in healthy volunteers who were dosed at precise 8- or 12-hour intervals.

The pharmacokinetics of nelfinavir are similar during BID and TID administration. In patients, the nelfinavir AUC_0-24 with 1250 mg BID administration was 52.8 ± 15.7 μg⋅h/ml (n=10) and with 750 mg TID administration was 43.6 ± 17.8 μg⋅h/ml (n=11). Trough drug exposures remain at least twenty fold greater than the mean IC₉₅ throughout the dosing interval for both regimens. The clinical relevance of relating in vitro measures to drug potency and clinical outcome has not been established. A greater than dose-proportional increase in nelfinavir plasma concentrations was observed after single doses; however, this was not observed after multiple dosing.

The absolute bioavailability of nelfinavir has not been determined.

Effect of Food on Oral Absorption

Food increases nelfinavir exposure and decreases nelfinavir pharmacokinetic variability relative to the fasted state. In one study, healthy volunteers received a single dose of 1250 mg of nelfinavir (5 × 250 mg tablets) under fasted or fed conditions (three meals with different caloric and fat contents). In a second study, healthy volunteers received single doses of 1250 mg nelfinavir (5 × 250 mg tablets) under fasted or fed conditions (two meals with different fat content). The results from the two studies are summarized below.

Increase in AUC, C_max and T_max for Nelfinavir in Fed State Relative to Fasted State Following 1250 mg nelfinavir (5 × 250 mg tablets):

Increase in Nelfinavir AUC, C_max and T_max in Fed Low Fat (20%) versus High fat (50%) State Relative to Fasted State Following 1250 mg nelfinavir (5 × 250 mg tablets):

Nelfinavir exposure increases with increasing calorie or fat content of meals taken with nelfinavir.

Distribution

Nelfinavir in serum is extensively protein-bound (≥98%). The estimated volumes of distribution in both animals and humans is 2-7 l/kg which exceeded total body water and suggests extensive penetration of nelfinavir into tissues.

Metabolism

In vitro studies demonstrated that multiple cytochrome P-450 isoforms including CYP3A, CYP2C19/C9 and CYP2D6 are responsible for the metabolism of nelfinavir. One major and several minor oxidative metabolites were found in plasma. The major oxidative metabolite, M8 (tert-butyl hydroxy nelfinavir), has in vitro antiviral activity equal to the parent drug and its formation is catalysed by the polymorphic cytochrome CYP2C19. The further degradation of M8 appears to be catalysed by CYP3A4. In subjects with normal CYP2C19 activity, plasma levels of this metabolite are approximately 25% of the total plasma nelfinavir-related concentration. It is expected that in CYP2C19 poor metabolisers or in patients receiving concomitantly strong CYP2C19 inhibitors, nelfinavir plasma levels would be elevated whereas levels of tert-butyl hydroxy nelfinavir would be negligible or non-measurable.

Elimination

Oral clearance estimates after single doses (24-33 l/h) and multiple doses (26-61 l/h) indicate that nelfinavir exhibits medium to high hepatic bioavailability. The terminal half-life in plasma was typically 3.5 to 5 hours. The majority (87%) of an oral 750 mg dose containing 14 C-nelfinavir was recovered in the faeces; total faecal radioactivity consisted of nelfinavir (22%) and numerous oxidative metabolites (78%). Only 1-2% of the dose was recovered in urine, of which unchanged nelfinavir was the major component.

Pharmacokinetics in special populations

Children

In children between the ages of 2 and 13 years, the clearance of orally administered nelfinavir is approximately 2 to 3 times higher than in adults, with large intersubject variability. Administration of nelfinavir oral powder or tablets at a dose of approximately 25-30 mg/kg TID with food achieves steady-state plasma concentrations that are similar to those achieved in adult patients receiving 750 mg TID.

The pharmacokinetics of nelfinavir have been investigated in 5 studies in paediatric patients from birth to 13 years of age. Patients received nelfinavir either three times daily or twice daily with food or with meals. The dosing regimens and associated AUC₂₄ values are summarized below.

Summary of Steady-state AUC₂₄ of nelfinavir in Paediatric Studies:

¹ Protocol specified dose (actual dose range)
² N: number of subjects with evaluable pharmacokinetic results
C_trough values are not presented in the table because they are not available from all studies

Pharmacokinetic data are also available for 86 patients (age 2 to 12 years) who received nelfinavir 25-35 mg/kg TID in Study AG1343-556. The pharmacokinetic data from Study AG1343-556 were more variable than data from other studies conducted in the paediatric population; the 95% confidence interval for AUC₂₄ was 9 to 121 mg.hr/L.

Overall, use of nelfinavir in the paediatric population is associated with highly variable drug exposure. The reason for this high variability is not known but may be due to inconsistent food intake in paediatric patients.

Elderly

There are no data available in the elderly.

Hepatic impairment

The multi-dose pharmacokinetics of nelfinavir have not been studied in HIV-positive patients with hepatic insufficiency.

Pharmacokinetics of nelfinavir after a single dose of 750 mg was studied in patients with liver impairment and healthy volunteers. A 49%-69% increase was observed in AUC of nelfinavir in the hepatically impaired groups with impairment (Child-Turcotte Classes A to C) compared to the healthy group. Specific dose recommendations for nelfinavir cannot be made based on the results of this study. A second study evaluated the steady state pharmacokinetics of nelfinavir (1250 mg twice daily for 2 weeks) in adult HIV-seronegative subjects with mild (Child-Pugh A; n=6) or moderate (Child-Pugh B; n=6) hepatic impairment. Compared to control subjects with normal hepatic function, the AUC and C_max of nelfinavir were not significantly different in subjects with mild impairment but were increased by 62% and 22%, respectively, in subjects with moderate hepatic impairment.

During in vitro studies, cloned human cardiac potassium channels (hERG) were inhibited by high concentrations of nelfinavir and its active metabolite M8. hERG potassium channels were inhibited by 20% at nelfinavir and M8 concentrations that are about four- to five-fold and seventy-fold, respectively, above the average free therapeutic levels in humans. By contrast, no effects suggesting prolongation of the QT-interval of the ECG were observed at similar doses in dogs or in isolated cardiac tissue. The clinical relevance of these in vitro data is unknown. However, based on data from products known to prolong the QT-interval, a block of hERG potassium channels of >20% may be clinically relevant. Therefore the potential for QT prolongation should be considered in cases of overdose (see section 4.9).

Acute and chronic toxicity

Oral acute and chronic toxicity studies were conducted in the mouse (500 mg/kg/day), rat (up to 1,000 mg/kg/day) and monkey (up to 800 mg/kg/day). There were increased liver weights and dose-related thyroid follicular cell hypertrophy in rats. Weight loss and general physical decline was observed in monkeys together with general evidence of gastrointestinal toxicity.

Mutagenicity

In vitro and in vivo studies with and without metabolic activation have shown that nelfinavir has no mutagenic or genotoxic activity.

Carcinogenicity

Two year oral carcinogenicity studies with nelfinavir mesilate were conducted in mice and rats. In mice, administration of up to 1000 mg/kg/day did not result in any evidence for an oncogenic effect. In rats administration of 1000 mg/kg/day resulted in increased incidences of thyroid follicular cell adenoma and carcinoma, relative to those for controls. Systemic exposures were 3 to 4 times those for humans given therapeutic doses. Administration of 300 mg/kg/day resulted in an increased incidence of thyroid follicular cell adenoma. Chronic nelfinavir treatment of rats has been demonstrated to produce effects consistent with enzyme induction, which predisposed rats, but not humans, to thyroid neoplasms. The weight of evidence indicates that nelfinavir is unlikely to be a carcinogen in humans.