Raltegravir

Raltegravir is an integrase strand transfer inhibitor active against the Human Immunodeficiency Virus (HIV-1). Raltegravir inhibits the catalytic activity of integrase, an HIV-encoded enzyme that is required for viral replication. Inhibition of integrase prevents the covalent insertion, or integration, of the HIV genome into the host cell genome. HIV genomes that fail to integrate cannot direct the production of new infectious viral particles, so inhibiting integration prevents propagation of the viral infection.

Antiviral activity in vitro

Raltegravir at concentrations of 31 ± 20 nM resulted in 95% inhibition (IC₉₅) of HIV-1 replication (relative to an untreated virus-infected culture) in human T-lymphoid cell cultures infected with the cell-line adapted HIV-1 variant H9IIIB. In addition, raltegravir inhibited viral replication in cultures of mitogen-activated human peripheral blood mononuclear cells infected with diverse, primary clinical isolates of HIV-1, including isolates from 5 non-B subtypes, and isolates resistant to reverse transcriptase inhibitors and protease inhibitors. In a single-cycle infection assay, raltegravir inhibited infection of 23 HIV isolates representing 5 non-B subtypes and 5 circulating recombinant forms with IC₅₀ values ranging from 5 to 12 nM.

Resistance

Most viruses isolated from patients failing raltegravir had high-level raltegravir resistance resulting from the appearance of two or more mutations in integrase. Most had a signature mutation at amino acid 155 (N155 changed to H), amino acid 148 (Q148 changed to H, K, or R), or amino acid 143 (Y143 changed to H, C, or R), along with one or more additional integrase mutations (e.g., L74M, E92Q, T97A, E138A/K, G140A/S, V151I, G163R, S230R). The signature mutations decrease viral susceptibility to raltegravir and addition of other mutations results in a further decrease in raltegravir susceptibility. Factors that reduced the likelihood of developing resistance included lower baseline viral load and use of other active anti-retroviral agents. Mutations conferring resistance to raltegravir generally also confer resistance to the integrase strand transfer inhibitor elvitegravir. Mutations at amino acid 143 confer greater resistance to raltegravir than to elvitegravir, and the E92Q mutation confers greater resistance to elvitegravir than to raltegravir. Viruses harbouring a mutation at amino acid 148, along with one or more other raltegravir resistance mutations, may also have clinically significant resistance to dolutegravir.

Absorption

As demonstrated in healthy volunteers administered single oral doses of raltegravir in the fasted state, raltegravir is rapidly absorbed with a tmax of approximately 3 hours postdose. Raltegravir AUC and C_max increase dose proportionally over the dose range 100 mg to 1,600 mg. Raltegravir C_12hr increases dose proportionally over the dose range of 100 to 800 mg and increases slightly less than dose proportionally over the dose range 100 mg to 1,600 mg. Dose proportionality has not been established in patients.

With twice-daily dosing, pharmacokinetic steady state is achieved rapidly, within approximately the first 2 days of dosing. There is little to no accumulation in AUC and C_max and evidence of slight accumulation in C_12hr. The absolute bioavailability of raltegravir has not been established.

Raltegravir may be administered with or without food. Raltegravir was administered without regard to food in the pivotal safety and efficacy studies in HIV-infected patients. Administration of multiple doses of raltegravir following a moderate-fat meal did not affect raltegravir AUC to a clinically meaningful degree with an increase of 13% relative to fasting. Raltegravir C_12hr was 66% higher and C_max was 5% higher following a moderate-fat meal compared to fasting. Administration of raltegravir following a high-fat meal increased AUC and C_max by approximately 2-fold and increased C_12hr by 4.1-fold. Administration of raltegravir following a low-fat meal decreased AUC and C_max by 46% and 52%, respectively; C_12hr was essentially unchanged. Food appears to increase pharmacokinetic variability relative to fasting.

Overall, considerable variability was observed in the pharmacokinetics of raltegravir. For observed C_12hr in BENCHMRK 1 and 2 the coefficient of variation (CV) for inter-subject variability = 212% and the CV for intra-subject variability = 122%. Sources of variability may include differences in co-administration with food and concomitant medicines.

Distribution

Raltegravir is approximately 83% bound to human plasma protein over the concentration range of 2 to 10 µM.

Raltegravir readily crossed the placenta in rats, but did not penetrate the brain to any appreciable extent.

In two studies of HIV-1 infected patients who received raltegravir 400 mg twice daily, raltegravir was readily detected in the cerebrospinal fluid. In the first study (n=18), the median cerebrospinal fluid concentration was 5.8% (range 1 to 53.5%) of the corresponding plasma concentration. In the second study (n=16), the median cerebrospinal fluid concentration was 3% (range 1 to 61%) of the corresponding plasma concentration. These median proportions are approximately 3- to 6-fold lower than the free fraction of raltegravir in plasma.

Biotransformation and excretion

The apparent terminal half-life of raltegravir is approximately 9 hours, with a shorter α-phase half-life (~1 hour) accounting for much of the AUC. Following administration of an oral dose of radiolabeled raltegravir, approximately 51 and 32% of the dose was excreted in faeces and urine, respectively. In faeces, only raltegravir was present, most of which is likely to be derived from hydrolysis of raltegravir-glucuronide secreted in bile as observed in preclinical species. Two components, namely raltegravir and raltegravir-glucuronide, were detected in urine and accounted for approximately 9 and 23% of the dose, respectively. The major circulating entity was raltegravir and represented approximately 70% of the total radioactivity; the remaining radioactivity in plasma was accounted for by raltegravir-glucuronide. Studies using isoform-selective chemical inhibitors and cDNA-expressed UDP-glucuronosyltransferases (UGT) show that UGT1A1 is the main enzyme responsible for the formation of raltegravir-glucuronide. Thus, the data indicate that the major mechanism of clearance of raltegravir in humans is UGT1A1-mediated glucuronidation.

UGT1A1 Polymorphism

In a comparison of 30 subjects with *28/*28 genotype to 27 subjects with wild-type genotype, the geometric mean ratio (90% CI) of AUC was 1.41 (0.96, 2.09) and the geometric mean ratio of C_12hr was 1.91 (1.43, 2.55). Dose adjustment is not considered necessary in subjects with reduced UGT1A1 activity due to genetic polymorphism.

Special populations

Paediatric population

Based on a formulation comparison study in healthy adult volunteers, the chewable tablet and granules for oral suspension have higher oral bioavailability compared to the 400 mg tablet. In this study, administration of the chewable tablet with a high fat meal led to an average 6% decrease in AUC, 62% decrease in C_max, and 188% increase in C_12hr compared to administration in the fasted state. Administration of the chewable tablet with a high fat meal does not affect raltegravir pharmacokinetics to a clinically meaningful degree and the chewable tablet can be administered without regard to food. The effect of food on the granules for oral suspension formulation was not studied.

Table 1 displays pharmacokinetic parameters in the 400 mg tablet, the chewable tablet, and the granules for oral suspension, by body weight.

Table 1. Raltegravir Pharmacokinetic Parameters IMPAACT P1066 Following Administration of Doses in Posology:

* Number of patients with intensive pharmacokinetic (PK) results at the final recommended dose.
^† Geometric coefficient of variation.

HIV-1 Exposed Neonates

IMPAACT P1110 is a Phase I trial to evaluate the safety and pharmacokinetics of raltegravir granules for suspension (GFS) with standard care PMTCT in full term HIV-1-exposed neonates. Cohort 1 (N=16, 10 exposed and 6 unexposed to raltegravir in utero) received 2 single doses of raltegravir GFS (within 48 hours and 7-10 days after birth); Cohort 2 (N=26, all raltegravir unexposed in utero) received raltegravir GFS for 6 weeks: 1.5 mg/kg once daily starting within 48 hours of birth through Week 1; 3 mg/kg twice daily Weeks 2 to 4; and 6 mg/kg twice daily Weeks 5 and 6.

Table 2 displays pharmacokinetic parameters for neonates in Cohort 2 at birth and at 2 weeks of age. Elimination of raltegravir in vivo in human is primarily through the UGT1A1-mediated glucuronidation pathway. UGT1A1 catalytic activity is negligible at birth and matures after birth. The dose recommended for neonates less than 4 weeks of age takes into consideration the rapidly increasing UGT1A1 activity and drug clearance from birth to 4 weeks of age.

Table 2. Raltegravir Pharmacokinetic Parameters IMPAACT P1110 Following Age and Weight Based Dosing of the Granules for Suspension:

* Number of patients with intensive pharmacokinetic (PK) results at the final recommended dose.
^† Geometric coefficient of variation.
^‡ AUC0-24hr (N=24), C24hr
^§ AUC0-12hr, C12hr

Elderly

There was no clinically meaningful effect of age on raltegravir pharmacokinetics in healthy subjects and patients with HIV-1 infection, over the age range studied (19 to 84 years, with few individuals over the age of 65).

Gender, race and BMI

There were no clinically important pharmacokinetic differences due to gender, race or body mass index (BMI) in adults.

Renal impairment

Renal clearance of unchanged medicinal product is a minor pathway of elimination. In adults, there were no clinically important pharmacokinetic differences between patients with severe renal insufficiency and healthy subjects. Because the extent to which raltegravir may be dialysable is unknown, dosing before a dialysis session should be avoided.

Hepatic impairment

Raltegravir is eliminated primarily by glucuronidation in the liver. In adults, there were no clinically important pharmacokinetic differences between patients with moderate hepatic insufficiency and healthy subjects. The effect of severe hepatic insufficiency on the pharmacokinetics of raltegravir has not been studied.

Non-clinical toxicology studies, including conventional studies of safety pharmacology, repeated-dose toxicity, genotoxicity, developmental toxicity and juvenile toxicity, have been conducted with raltegravir in mice, rats, dogs and rabbits. Effects at exposure levels sufficiently in excess of clinical exposure levels indicate no special hazard for humans.

Mutagenicity

No evidence of mutagenicity or genotoxicity was observed in in vitro microbial mutagenesis (Ames) tests, in vitro alkaline elution assays for DNA breakage and in vitro and in vivo chromosomal aberration studies.

Carcinogenicity

A carcinogenicity study of raltegravir in mice did not show any carcinogenic potential. At the highest dose levels, 400 mg/kg/day in females and 250 mg/kg/day in males, systemic exposure was similar to that at the clinical dose of 400 mg twice daily. In rats, tumours (squamous cell carcinoma) of the nose/nasopharynx were identified at 300 and 600 mg/kg/day in females and at 300 mg/kg/day in males. This neoplasia could result from local deposition and/or aspiration of drug on the mucosa of the nose/nasopharynx during oral gavage dosing and subsequent chronic irritation and inflammation; it is likely that it is of limited relevance for the intended clinical use. At the NOAEL, systemic exposure was similar to that at the clinical dose of 400 mg twice daily. Standard genotoxicity studies to evaluate mutagenicity and clastogenicity were negative.

Developmental toxicity

Raltegravir was not teratogenic in developmental toxicity studies in rats and rabbits. A slight increase in incidence of supernumerary ribs, a variant in the normal developmental process, was observed in rat foetuses of dams exposed to raltegravir at approximately 4.4-fold human exposure at 400 mg twice daily based on AUC_0-24hr. No development effects were seen at 3.4-fold human exposure at 400 mg twice daily based on AUC_0-24hr. Similar findings were not observed in rabbits.