Darunavir and Cobicistat

Darunavir is an inhibitor of the dimerisation and of the catalytic activity of the HIV-1 protease (K_D of 4.5 × 10^-12M). It selectively inhibits the cleavage of HIV encoded Gag-Pol polyproteins in virus infected cells, thereby preventing the formation of mature infectious virus particles.

Cobicistat is a mechanism-based inhibitor of cytochromes P450 of the CYP3A subfamily. Inhibition of CYP3A-mediated metabolism by cobicistat enhances the systemic exposure of CYP3A substrates, such as darunavir, where bioavailability is limited and half-life is shortened due to CYP3A-dependent metabolism.

Resistance

In vitro selection of darunavir-resistant virus from wild type HIV-1 was lengthy (>3 years). The selected viruses were unable to grow in the presence of darunavir concentrations above 400 nM. Viruses selected in these conditions and showing decreased susceptibility to darunavir (range: 23-50-fold) harboured 2 to 4 amino acid substitutions in the protease gene. The decreased susceptibility to darunavir of the emerging viruses in the selection experiment could not be explained by the emergence of these protease mutations.

The resistance profile of darunavir/cobicistat is driven by darunavir. Cobicistat does not select any HIV resistance mutations, due to its lack of antiviral activity. The resistance profile of darunavir/cobicistat is supported by two Phase III trials conducted with darunavir/ritonavir in treatment-naïve (ARTEMIS) and treatment-experienced (ODIN) patients and the analysis of 48 week data from trial GS-US-216-130 in treatment-naïve and treatment-experienced patients.

Low rates of developing resistant HIV-1 virus were observed in ART-naïve patients who are treated for the first time with darunavir/cobicistat or darunavir/ritonavir 800/100 mg once daily in combination with other ART, and in ART-experienced patients with no darunavir RAMs receiving darunavir/cobicistat or darunavir/ritonavir 800/100 mg once daily in combination with other ART. The table below shows the development of HIV-1 protease mutations and loss of susceptibility to HIV PIs in virologic failures at endpoint in the GS-US-216-130, ARTEMIS and ODIN trials.

^a Virologic failures selected for resistance testing were defined as: never suppressed: HIV-1 RNA <1 log₁₀ reduction from baseline and ≥50 copies/mL at week 8, confirmed at the following visit; rebound: HIV-1 RNA <50 copies/mL followed by confirmed HIV-1 RNA to ≥400 copies/mL or confirmed >1 log₁₀ HIV-1 RNA increase from the nadir; discontinuations with HIV-1 RNA ≥400 copies/mL at last visit
^b Virologic failures based on TLOVR non-VF censored algorithm (HIV-1 RNA >50 copies/mL)
^c IAS-USA lists
^d In GS-US-216-130 baseline phenotype was not available

Cross-resistance

In the virologic failures of the GS-US-216-130 trial no cross-resistance with other HIV PIs was observed. Refer to the table above for information on ARTEMIS and ODIN.

Darunavir exposure was shown to be comparable in a bioavailability trial between darunavir/cobicistat and darunavir/ritonavir 800/100 mg q.d. at steady-state and fed conditions in healthy subjects. The bioequivalence between darunavir/cobicistat and darunavir/cobicistat 800/150 mg co-administered as single agents was established under fed and fasted conditions in healthy subjects.

Absorption

Darunavir

The absolute oral bioavailability of a single 600 mg dose of darunavir alone is approximately 37%.

Darunavir was rapidly absorbed following oral administration of darunavir/cobicistat in healthy volunteers. Maximum plasma concentration of darunavir in the presence of cobicistat is generally achieved within 3 to 4.5 hours. Following oral administration of darunavir/cobicistat in healthy volunteers, maximum plasma concentrations of cobicistat were observed 2 to 5 hours post-dose.

When administered with food, the relative exposure of darunavir is 1.7-fold higher as compared to intake without food. Therefore, darunavir/cobicistat tablets should be taken with food. The type of food does not affect exposure to darunavir/cobicistat.

Distribution

Darunavir

Darunavir is approximately 95% bound to plasma protein. Darunavir binds primarily to plasma α1-acid glycoprotein.

Following intravenous administration, the volume of distribution of darunavir alone was 88.1 ± 59.0 L (Mean ± SD) and increased to 131 ± 49.9 L (Mean ± SD) in the presence of 100 mg twice-daily ritonavir.

Cobicistat

Cobicistat is 97 to 98% bound to human plasma proteins and the mean plasma to blood-drug concentration ratio was approximately 2.

Biotransformation

Darunavir

In vitro experiments with human liver microsomes (HLMs) indicate that darunavir primarily undergoes oxidative metabolism. Darunavir is extensively metabolised by the hepatic CYP system and almost exclusively by isozyme CYP3A4. A ¹⁴C-darunavir trial in healthy volunteers showed that a majority of the radioactivity in plasma after a single 400/100 mg darunavir with ritonavir dose was due to the parent active substance. At least 3 oxidative metabolites of darunavir have been identified in humans; all showed activity that was at least 10-fold less than the activity of darunavir against wild type HIV.

Cobicistat

Cobicistat is metabolised via CYP3A (major)- and CYP2D6 (minor)-mediated oxidation and does not undergo glucuronidation. Following oral administration of ¹⁴C-cobicistat, 99% of circulating radioactivity in plasma was unchanged cobicistat. Low levels of metabolites are observed in urine and faeces and do not contribute to the CYP3A inhibitory activity of cobicistat.

Elimination

Darunavir

After a 400/100 mg ¹⁴C-darunavir with ritonavir dose, approximately 79.5% and 13.9% of the administered dose of ¹⁴C-darunavir could be retrieved in faeces and urine, respectively. Unchanged darunavir accounted for approximately 41.2% and 7.7% of the administered dose in faeces and urine, respectively. The terminal elimination half-life of darunavir was approximately 15 hours when combined with ritonavir. The intravenous clearance of darunavir alone (150 mg) and in the presence of low dose ritonavir was 32.8 L/h and 5.9 L/h, respectively.

Cobicistat

Following oral administration of ¹⁴C-cobicistat, 86% and 8.2% of the dose were recovered in faeces and urine, respectively. The median terminal plasma half-life of cobicistat following administration of darunavir/cobicistat is approximately 3-4 hours.

Special populations

Paediatric population

Available pharmacokinetic data for the different components of darunavir/cobicistat indicate there were no clinically relevant differences in exposure between adults and adolescents. In addition, the pharmacokinetics of darunavir 800 mg co-administered with cobicistat 150 mg in paediatric patients have been studied in 7 adolescents aged 12 to less than 18 years, weighing at least 40 kg who received darunavir 800 mg co-administered with cobicistat 150 mg in Study GS-US-216-0128. The geometric mean adolescent exposure (AUC_tau) was similar for darunavir and increased 19% for cobicistat compared to exposures achieved in adults who received darunavir 800 mg co-administered with cobicistat 150 mg in Study GS-US-216-0130. The difference observed for cobicistat was not considered clinically relevant.

Elderly

Darunavir:

Limited information is available in this population. Population pharmacokinetic analysis in HIV infected patients showed that darunavir pharmacokinetics are not considerably different in the age range (18 to 75 years) evaluated in HIV infected patients (n=12, age 65 years). However, only limited data were available in patients above the age of 65 years.

Cobicistat:

Pharmacokinetics of cobicistat have not been fully evaluated in older people (65 years of age and older).

Gender

Darunavir:

Population pharmacokinetic analysis showed a slightly higher darunavir exposure (16.8%) in HIV infected females compared to males. This difference is not clinically relevant.

Cobicistat:

No clinically relevant pharmacokinetic differences due to gender have been identified for cobicistat.

Renal impairment

Darunavir/cobicistat has not been investigated in patients with renal impairment.

Darunavir:

Results from a mass balance study with ¹⁴C-darunavir with ritonavir showed that approximately 7.7% of the administered dose of darunavir is excreted in the urine unchanged.

Although darunavir has not been studied in patients with renal impairment, population pharmacokinetic analysis showed that the pharmacokinetics of darunavir were not significantly affected in HIV infected patients with moderate renal impairment (CrCl between 30-60 mL/min, n=20).

Cobicistat:

A trial of the pharmacokinetics of cobicistat was performed in non-HIV-1 infected subjects with severe renal impairment (estimated creatinine clearance below 30 mL/min). No meaningful differences in cobicistat pharmacokinetics were observed between subjects with severe renal impairment and healthy subjects, consistent with low renal clearance of cobicistat.

Hepatic impairment

Darunavir/cobicistat has not been investigated in patients with hepatic impairment.

Darunavir:

Darunavir is primarily metabolised and eliminated by the liver. In a multiple dose trial with darunavir/ritonavir (600/100 mg) twice daily, it was demonstrated that the total plasma concentrations of darunavir in subjects with mild (Child-Pugh Class A, n=8) and moderate (Child-Pugh Class B, n=8) hepatic impairment were comparable with those in healthy subjects. However, unbound darunavir concentrations were approximately 55% (Child-Pugh Class A) and 100% (Child-Pugh Class B) higher, respectively. The clinical relevance of this increase is unknown, therefore, darunavir/ritonavir should be used with caution. The effect of severe hepatic impairment on the pharmacokinetics of darunavir has not been studied.

Cobicistat:

Cobicistat is primarily metabolised and eliminated by the liver. A trial of the pharmacokinetics of cobicistat was performed in non-HIV-1 infected subjects with moderate hepatic impairment (Child-Pugh Class B). No clinically relevant differences in cobicistat pharmacokinetics were observed between subjects with moderate impairment and healthy subjects. No dosage adjustment of darunavir/cobicistat is necessary for patients with mild to moderate hepatic impairment. The effect of severe hepatic impairment (Child-Pugh Class C) on the pharmacokinetics of cobicistat has not been studied.

Hepatitis B and/or hepatitis C virus co-infection

There were insufficient pharmacokinetic data in the clinical trials to determine the effect of hepatitis B and/or C virus infection on the pharmacokinetics of darunavir and cobicistat.

Pregnancy and postpartum

Treatment with darunavir/cobicistat during pregnancy results in low darunavir exposure. In women receiving darunavir/cobicistat during the second trimester of pregnancy, mean intra-individual values for total darunavir C_max, AUC_24h and C_min were 49%, 56% and 92% lower, respectively, as compared with postpartum; during the third trimester of pregnancy, total darunavir C_max, AUC_24h and C_min values were 37%, 50% and 89% lower, respectively, as compared with postpartum. The unbound fraction was also substantially reduced, including around 90% reductions of C_min levels. The main cause of these low exposures is a marked reduction in cobicistat exposure as a consequence of pregnancy-associated enzyme induction (see below).

Pharmacokinetic results of total darunavir after administration of darunavir/cobicistat 800/150 mg once daily as part of an antiretroviral regimen, during the second trimester of pregnancy, the third trimester of pregnancy, and postpartum:

The exposure to cobicistat was lower during pregnancy, potentially leading to suboptimal boosting of darunavir. During the second trimester of pregnancy, cobicistat C_max, AUC24h, and C_min were 50%, 63%, and 83% lower, respectively, as compared with postpartum. During the third trimester of pregnancy, cobicistat C_max, AUC_24h, and C_min, were 27%, 49%, and 83% lower, respectively, as compared with postpartum.

Darunavir

Animal toxicology studies have been conducted at exposures up to clinical exposure levels with darunavir alone, in mice, rats and dogs and in combination with ritonavir in rats and dogs.

In repeated-dose toxicology studies in mice, rats and dogs, there were only limited effects of treatment with darunavir. In rodents the target organs identified were the haematopoietic system, the blood coagulation system, liver and thyroid. A variable but limited decrease in red blood cell-related parameters was observed, together with increases in activated partial thromboplastin time. Changes were observed in liver (hepatocyte hypertrophy, vacuolation, increased liver enzymes) and thyroid (follicular hypertrophy). In the rat, the combination of darunavir with ritonavir lead to a small increase in effect on RBC parameters, liver and thyroid and increased incidence of islet fibrosis in the pancreas (in male rats only) compared to treatment with darunavir alone. In the dog, no major toxicity findings or target organs were identified up to exposures equivalent to clinical exposure at the recommended dose.

In a study conducted in rats, the number of corpora lutea and implantations were decreased in the presence of maternal toxicity. Otherwise, there were no effects on mating or fertility with darunavir treatment up to 1,000 mg/kg/day and exposure levels below (AUC-0.5 fold) of that in human at the clinically recommended dose. Up to same dose levels, there was no teratogenicity with darunavir in rats and rabbits when treated alone nor in mice when treated in combination with ritonavir. The exposure levels were lower than those with the recommended clinical dose in humans. In a pre- and postnatal development assessment in rats, darunavir with and without ritonavir, caused a transient reduction in body weight gain of the offspring pre-weaning and there was a slight delay in the opening of eyes and ears. Darunavir in combination with ritonavir caused a reduction in the number of pups that exhibited the startle response on day 15 of lactation and a reduced pup survival during lactation. These effects may be secondary to pup exposure to the active substance via the milk and/or maternal toxicity. No post weaning functions were affected with darunavir alone or in combination with ritonavir. In juvenile rats receiving darunavir up to days 23-26, increased mortality was observed with convulsions in some animals. Exposure in plasma, liver and brain was considerably higher than in adult rats after comparable doses in mg/kg between days 5 and 11 of age. After day 23 of life, the exposure was comparable to that in adult rats. The increased exposure was likely at least partly due to immaturity of the drug-metabolising enzymes in juvenile animals. No treatment related mortalities were noted in juvenile rats dosed at 1,000 mg/kg darunavir (single dose) on day 26 of age or at 500 mg/kg (repeated dose) from day 23 to 50 of age, and the exposures and toxicity profile were comparable to those observed in adult rats.

Due to uncertainties regarding the rate of development of the human blood brain barrier and liver enzymes darunavir/cobicistat should not be used in paediatric patients below 3 years of age.

Darunavir was evaluated for carcinogenic potential by oral gavage administration to mice and rats up to 104 weeks. Daily doses of 150, 450 and 1,000 mg/kg were administered to mice and doses of 50, 150 and 500 mg/kg were administered to rats. Dose-related increases in the incidences of hepatocellular adenomas and carcinomas were observed in males and females of both species. Thyroid follicular cell adenomas were noted in male rats. Administration of darunavir did not cause a statistically significant increase in the incidence of any other benign or malignant neoplasm in mice or rats. The observed hepatocellular and thyroid tumours in rodents are considered to be of limited relevance to humans. Repeated administration of darunavir to rats caused hepatic microsomal enzyme induction and increased thyroid hormone elimination, which predispose rats, but not humans, to thyroid neoplasms. At the highest tested doses, the systemic exposures (based on AUC) to darunavir when co-administered with ritonavir were between 0.4- and 0.7-fold (mice) and 0.7- and 1-fold (rats), relative to those observed in humans at the recommended therapeutic doses.

After 2 years administration of darunavir at exposures at or below the human exposure, kidney changes were observed in mice (nephrosis) and rats (chronic progressive nephropathy).

Darunavir was not mutagenic or genotoxic in a battery of in vitro and in vivo assays including bacterial reverse mutation (Ames), chromosomal aberration in human lymphocytes and in vivo micronucleus test in mice.

Cobicistat

Non-clinical data reveal no special hazard for humans based on conventional studies of repeated dose toxicity, genotoxicity, and toxicity to reproduction and development. No teratogenic effects were observed in rats and rabbit developmental toxicity studies. In rats, ossification changes in the spinal column and sternebrae of fetuses occurred at a dose that produced significant maternal toxicity.

Ex vivo rabbit studies and in vivo dog studies suggest that cobicistat has a low potential for QT prolongation, and may slightly prolong the PR interval and decrease left ventricular function at mean concentrations at least 10-fold higher than the human exposure at the recommended 150 mg daily dose.

A long term carcinogenicity study of cobicistat in rats revealed tumourigenic potential specific for this species, that is regarded as of no relevance for humans. A long term carcinogenicity study in mice did not show any carcinogenic potential.