Saquinavir

Mechanism of action

The HIV protease is an essential viral enzyme required for the specific cleavage of viral gag and gag-pol polyproteins. Saquinavir selectively inhibits the HIV protease, thereby preventing the creation of mature infectious virus particles.

Saquinavir is essentially completely metabolised by CYP3A4. Ritonavir inhibits the metabolism of saquinavir, thereby increasing (“boosting”) the plasma levels of saquinavir.

Absorption

In HIV-infected adult patients, saquinavir in combination with ritonavir at doses of 1000/100 mg twice daily provides saquinavir systemic exposures over a 24-hour period similar to or greater than those achieved with saquinavir soft capsules 1200 mg tid (see Table 1). The pharmacokinetics of saquinavir is stable during long-term treatment.

Table 1. Mean (% CV) AUC, C_max and C_min of saquinavir in patients following multiple dosing of saquinavir, saquinavir soft capsules, saquinavir/ritonavir, and saquinavir soft capsules/ritonavir:

Absolute bioavailability averaged 4% (CV 73%, range: 1% to 9%) in 8 healthy volunteers who received a single 600 mg dose (3 × 200 mg hard capsule) of saquinavir following a heavy breakfast. The low bioavailability is thought to be due to a combination of incomplete absorption and extensive firstpass metabolism. Gastric pH has been shown to be only a minor component in the large increase in bioavailability seen when given with food. The absolute bioavailability of saquinavir co-administered with ritonavir has not been established in humans.

In combination with ritonavir, bioequivalence of saquinavir hard capsules and film-coated tablets was demonstrated under fed conditions.

Effective therapy in treatment naïve patients is associated with a C_min of approximately 50 ng/ml and an AUC_0-24 of about 20,000 ng·h/ml. Effective therapy in treatment experienced patients is associated with a C_min of approximately 100 ng/ml and an AUC_0-24 of about 20,000 ng·h/ml.

In treatment-naïve HIV-1 infected patients initiating saquinavir/ritonavir treatment with a modified saquinavir/ritonavir dosing regimen of saquinavir 500 mg two times daily with ritonavir 100 mg two times daily for the first 7 days of treatment and increased to saquinavir 1000 mg two times daily with ritonavir 100 mg two times daily in the subsequent 7 days, saquinavir systemic exposures generally approached or exceeded the range of historical steady-state values with the standard saquinavir/ritonavir 1000 mg/100 mg bid dosing regimen across study days (see Tables 2 and 1).

Table 2: Mean (CV%) PK Parameters following administration of the Modified saquinavir/ritonavir Regimen in Treatment Naïve HIV-1 infected Patients initiating treatment with saquinavir/ritonavir

In vitro studies have shown that saquinavir is a substrate for P-glycoprotein (P-gp).

Effect of food

In a cross-over study in 22 HIV-infected patients treated with saquinavir/ritonavir 1000 mg/100 mg twice daily and receiving three consecutive doses under fasting conditions or after a high-fat, high-calorie meal (46 g fat, 1,091 Kcal), the AUC_0-12, C_max and Ctrough values of saquinavir under fasting conditions were about 70 per cent lower than with a high-fat meal. All but one of the patients achieved Ctrough values of saquinavir above the therapeutic threshold (100 ng/ml) in the fasted state. There were no clinically significant differences in the pharmacokinetic profile of ritonavir in fasting and fed conditions but the ritonavir Ctrough (geometric mean 245 vs. 348 ng/ml) was lower in the fasting state compared to the administration with a meal. Saquinavir/ritonavir should be administered with or after food.

Distribution in adults

Saquinavir partitions extensively into the tissues. The mean steady-state volume of distribution following intravenous administration of a 12 mg dose of saquinavir was 700 l (CV 39%). It has been shown that saquinavir is approximately 97% bound to plasma proteins up to 30 µg/ml. In two patients receiving saquinavir 600 mg three times daily, cerebrospinal fluid concentrations of saquinavir were negligible when compared to concentrations from matching plasma samples.

Biotransformation and elimination in adults

In vitro studies using human liver microsomes have shown that the metabolism of saquinavir is cytochrome P450 mediated with the specific isoenzyme, CYP3A4, responsible for more than 90% of the hepatic metabolism. Based on in vitro studies, saquinavir is rapidly metabolised to a range of mono- and di-hydroxylated inactive compounds. In a mass balance study using 600 mg ¹⁴C-saquinavir (n=8), 88% and 1% of the orally administered radioactivity, was recovered in faeces and urine, respectively, within 4 days of dosing. In an additional four subjects administered 10.5 mg ¹⁴C-saquinavir intravenously, 81% and 3% of the intravenously administered radioactivity was recovered in faeces and urine, respectively, within 4 days of dosing. 13% of circulating saquinavir in plasma was present as unchanged compound after oral administration and the remainder as metabolites. Following intravenous administration 66% of circulating saquinavir was present as unchanged compound and the remainder as metabolites, suggesting that saquinavir undergoes extensive first pass metabolism. In vitro experiments have shown that the hepatic metabolism of saquinavir becomes saturable at concentrations above 2 µg/ml.

Systemic clearance of saquinavir was high, 1.14 l/h/kg (CV 12%), slightly above the hepatic plasma flow, and constant after intravenous doses of 6, 36 and 72 mg. The mean residence time of saquinavir was 7 hours (n=8).

Special populations

Effect of gender following treatment with saquinavir/ritonavir

A gender difference was observed with females showing higher saquinavir exposure than males (AUC on average 56% higher and C_max on average 26% higher) in the bioequivalence study comparing saquinavir 500 mg film coated tablets with saquinavir 200 mg hard capsules both in combination with ritonavir. There was no evidence that age and body-weight explained the gender difference in this study. Limited data from controlled clinical studies with the approved dosage regimen do not indicate a major difference in the efficacy and safety profile between men and women.

Patients with hepatic impairment

The effect of hepatic impairment on the steady state pharmacokinetics of saquinavir/ritonavir (1000 mg/100 mg twice daily for 14 days) was investigated in 7 HIV-infected patients with moderate liver impairment (Child Pugh Grade B score 7 to 9). The study included a control group consisting of 7 HIV-infected patients with normal hepatic function matched with the hepatically impaired patients for age, gender, weight and tobacco use. The mean (% coefficient of variation in parentheses) values for saquinavir AUC_0-12 and C_max were 24.3 (102%) µg·hr/ml and 3.6 (83%) µg/ml, respectively, for HIV-infected patients with moderate hepatic impairment. The corresponding values in the control group were 28.5 (71%) µg·hr/ml and 4.3 (68%) µg/ml. The geometric mean ratio (ratio of pharmacokinetic parameters in hepatically impaired patients to patients with normal liver function) (90% confidence interval) was 0.7 (0.3 to 1.6) for both AUC_0-12 and C_max, which suggests approximately 30% reduction in the pharmacokinetic exposure in patients with moderate hepatic impairment. Results are based on total concentrations (protein-bound and unbound). Concentrations unbound at steady-state were not assessed. No dosage adjustment seems warranted for patients with moderate hepatic impairment based on limited data. Close monitoring of safety (including signs of cardiac arrhythmia) and of virologic response is recommended due to increased variability of the exposure in this population.

Paediatric Patients

Steady state pharmacokinetic information is available from HIV-infected paediatric patients from study NV20911. In this study, 5 patients were <2 years and 13 between 2 to <6 years and received 50 mg/kg saquinavir bid (not to exceed 1000 mg bid) boosted with ritonavir at 3 mg/kg for patients with body weight ranging from 5 to <15 kg or 2.5 mg/kg for patients with body weight ranging from 15 to 40 kg (not to exceed 100 mg bid). Sixteen of 18 children could not swallow saquinavir hard capsules and received medication by opening the capsules and mixing the contents with different vehicles. The pharmacokinetic exposure parameters for the “High Age Group” are listed in Table 3. Results of the “Low Age Group” are not shown as data are limited due to the small size of the group.

Table 3. Pharmacokinetic parameters of saquinavir at steady-state in HIV-infected pediatric patients:

|\3_. \3<>_.Mean ± SD Saquinavir (%CV)
Pharmacokinetic Parameters* |

* All parameters normalized to a 50 mg/kg dose

Steady state saquinavir exposures observed in paediatric trials were substantially higher than historical data in adults where dose- and exposure-dependent QTc and PR prolongation were observed.

Acute and chronic toxicity

Saquinavir was well tolerated in oral acute and chronic toxicity studies in mice, rats, dogs and marmosets.

Mutagenesis

Mutagenicity and genotoxicity studies, with and without metabolic activation where appropriate, have shown that saquinavir has no mutagenic activity in vitro in either bacterial (Ames test) or mammalian cells (Chinese hamster lung V79/HPRT test). Saquinavir does not induce chromosomal damage in vivo in the mouse micronucleus assay or in vitro in human peripheral blood lymphocytes and does not induce primary DNA damage in vitro in the unscheduled DNA synthesis test.

Carcinogenesis

There was no evidence of carcinogenic activity after the administration of saquinavir mesilate for 96 to 104 weeks to rats and mice. The plasma exposures (AUC values) in rats (maximum dose 1000 mg/kg/day) and in mice (maximum dose 2500 mg/kg/day) were lower than the expected plasma exposures obtained in humans at the recommended clinical dose of ritonavir boosted Invirase.

Reproductive toxicity

Fertility, peri- and postnatal development were not affected, and embryotoxic/teratogenic effects were not observed in rats or rabbits at plasma exposures lower than those achieved in humans at the recommended clinical dose of ritonavir boosted Invirase. Distribution studies in these species showed that the placental transfer of saquinavir is low (less than 5% of maternal plasma concentrations).

Safety pharmacology

Cloned human cardiac potassium channel (hERG) trafficking in vitro was inhibited by 75% at 30μM of saquinavir. Saquinavir inhibited both hERG current and L-type Ca++ channel current with respective IC₅₀ of 4.7 and 6.3 μM. In a myocardial distribution study in the rat an approximately 2-fold accumulation of saquinavir was observed in the heart compared to plasma after coadministration of saquinavir and ritonavir. The clinical relevance of these preclinical results are unknown, however cardiac conduction and repolarisation abnormalities in humans have been observed with saquinavir and ritonavir combination therapy.