Tenofovir alafenamide

Tenofovir alafenamide is a phosphonamidate prodrug of tenofovir (2'-deoxyadenosine monophosphate analogue). Tenofovir alafenamide enters primary hepatocytes by passive diffusion and by the hepatic uptake transporters OATP1B1 and OATP1B3. Tenofovir alafenamide is primarily hydrolysed to form tenofovir by carboxylesterase 1 in primary hepatocytes. Intracellular tenofovir is subsequently phosphorylated to the pharmacologically active metabolite tenofovir diphosphate. Tenofovir diphosphate inhibits HBV replication through incorporation into viral DNA by the HBV reverse transcriptase, which results in DNA chain termination.

Tenofovir has activity that is specific to HBV and HIV (HIV-1 and HIV-2). Tenofovir diphosphate is a weak inhibitor of mammalian DNA polymerases that include mitochondrial DNA polymerase γ and there is no evidence of mitochondrial toxicity in vitro based on several assays including mitochondrial DNA analyses.

Antiviral activity

The antiviral activity of tenofovir alafenamide was assessed in HepG2 cells against a panel of HBV clinical isolates representing genotypes A-H. The EC₅₀ (50% effective concentration) values for tenofovir alafenamide ranged from 34.7 to 134.4 nM, with an overall mean EC₅₀ of 86.6 nM. The CC₅₀ (50% cytotoxicity concentration) in HepG2 cells was >44,400 nM.

Resistance

In patients receiving tenofovir alafenamide, sequence analysis was performed on paired baseline and on-treatment HBV isolates for patients who either experienced virologic breakthrough (2 consecutive visits with HBV DNA ≥69 IU/mL after having been <69 IU/mL, or 1.0 log10 or greater increase in HBV DNA from nadir) or patients with HBV DNA ≥69 IU/mL at Week 48, or Week 96 or at early discontinuation at or after Week 24.

In a pooled analysis of patients receiving tenofovir alafenamide in Study 108 and Study 110 at Week 48 (N=20) and Week 96 (N=72), no amino acid substitutions associated with resistance to tenofovir alafenamide were identified in these isolates (genotypic and phenotypic analyses).

In virologically suppressed patients receiving tenofovir alafenamide following switch from tenofovir disoproxil treatment in Study 4018, through 96 weeks of tenofovir alafenamide treatment one patient in the tenofovir alafenamide-tenofovir alafenamide group experienced a virologic blip (one visit with HBV DNA ≥69 IU/mL) and one patient in the tenofovir disoproxil-tenofovir alafenamide group experienced a virologic breakthrough. No HBV amino acid substitutions associated with resistance to tenofovir alafenamide or tenofovir disoproxil were detected through 96 weeks of treatment.

In paediatric Study 1092, 30 patients aged 12 to <18 years and 9 patients aged 6 to <12 years receiving tenofovir alafenamide qualified for resistance analysis at Week 24. No HBV amino acid substitutions associated with resistance to tenofovir alafenamide were detected through 24 weeks of treatment. At Week 48, 31 patients aged 12 to <18 years and 12 patients aged 6 to <12 years qualified for resistance analysis (both tenofovir alafenamide group and placebo roll over to tenofovir alafenamide group at Week 24). No HBV amino acid substitutions associated with resistance to tenofovir alafenamide were detected through 48 weeks of treatment.

Cross-resistance

The antiviral activity of tenofovir alafenamide was evaluated against a panel of isolates containing nucleos(t)ide reverse transcriptase inhibitor mutations in HepG2 cells. HBV isolates expressing the rtV173L, rtL180M, and rtM204V/I substitutions associated with resistance to lamivudine remained susceptible to tenofovir alafenamide (<2-fold change in EC₅₀). HBV isolates expressing the rtL180M, rtM204V plus rtT184G, rtS202G, or rtM250V substitutions associated with resistance to entecavir remained susceptible to tenofovir alafenamide. HBV isolates expressing the rtA181T, rtA181V, or rtN236T single substitutions associated with resistance to adefovir remained susceptible to tenofovir alafenamide; however, the HBV isolate expressing rtA181V plus rtN236T exhibited reduced susceptibility to tenofovir alafenamide (3.7-fold change in EC₅₀). The clinical relevance of these substitutions is not known.

Absorption

Following oral administration of tenofovir alafenamide under fasted conditions in adult patients with chronic hepatitis B, peak plasma concentrations of tenofovir alafenamide were observed approximately 0.48 hours post-dose. Based on Phase 3 population pharmacokinetic analysis in patients with chronic hepatitis B, mean steady state AUC_0-24 for tenofovir alafenamide (N=698) and tenofovir (N=856) were 0.22 µg•h/mL and 0.32 µg•h/mL, respectively. Steady state C_max for tenofovir alafenamide and tenofovir were 0.18 and 0.02 µg/mL, respectively. Relative to fasting conditions, the administration of a single dose of tenofovir alafenamide with a high fat meal resulted in a 65% increase in tenofovir alafenamide exposure.

Distribution

The binding of tenofovir alafenamide to human plasma proteins in samples collected during clinical studies was approximately 80%. The binding of tenofovir to human plasma proteins is less than 0.7% and is independent of concentration over the range of 0.01-25 µg/mL.

Biotransformation

Metabolism is a major elimination pathway for tenofovir alafenamide in humans, accounting for >80% of an oral dose. In vitro studies have shown that tenofovir alafenamide is metabolised to tenofovir (major metabolite) by carboxylesterase-1 in hepatocytes; and by cathepsin A in peripheral blood mononuclear cells (PBMCs) and macrophages. In vivo, tenofovir alafenamide is hydrolysed within cells to form tenofovir (major metabolite), which is phosphorylated to the active metabolite, tenofovir diphosphate.

In vitro, tenofovir alafenamide is not metabolised by CYP1A2, CYP2C8, CYP2C9, CYP2C19, or CYP2D6. Tenofovir alafenamide is minimally metabolised by CYP3A4.

Elimination

Renal excretion of intact tenofovir alafenamide is a minor pathway with <1% of the dose eliminated in urine. Tenofovir alafenamide is mainly eliminated following metabolism to tenofovir. Tenofovir alafenamide and tenofovir have a median plasma half-life of 0.51 and 32.37 hours, respectively. Tenofovir is renally eliminated from the body by the kidneys by both glomerular filtration and active tubular secretion.

Linearity/non-linearity

Tenofovir alafenamide exposures are dose proportional over the dose range of 8 to 125 mg.

Pharmacokinetics in special populations

Age, gender and ethnicity

No clinically relevant differences in pharmacokinetics according to age or ethnicity have been identified. Differences in pharmacokinetics according to gender were not considered to be clinically relevant.

Hepatic impairment

In patients with severe hepatic impairment, total plasma concentrations of tenofovir alafenamide and tenofovir are lower than those seen in patients with normal hepatic function. When corrected for protein binding, unbound (free) plasma concentrations of tenofovir alafenamide in severe hepatic impairment and normal hepatic function are similar.

Renal impairment

No clinically relevant differences in tenofovir alafenamide or tenofovir pharmacokinetics were observed between healthy patients and patients with severe renal impairment (estimated CrCl >15 but <30 mL/min) in studies of tenofovir alafenamide (Table 1).

Exposures of tenofovir in patients with ESRD (estimated creatinine clearance <15 mL/min) on chronic haemodialysis who received tenofovir alafenamide (N=5) were substantially higher than in patients with normal renal function (Table 1). No clinically relevant differences in tenofovir alafenamide pharmacokinetics were observed in patients with ESRD on chronic haemodialysis as compared to those with normal renal function.

Table 1. Pharmacokinetics of Tenofovir Alafenamide and its Metabolite Tenofovir in Patients with Renal Impairment as Compared to Patients with Normal Renal Function:

CV = coefficient of variation
^a By Cockcroft-Gault method.
^b PK assessed on a single dose of tenofovir alafenamide 25 mg in patients with normal renal function and in patients with severe renal impairment in Study GS-US-120-0108.
^c PK assessed prior to haemodialysis following multiple-dose administration of tenofovir alafenamide 25 mg in 5 HBVinfected patients in Study GS-US-320-4035. These patients had a median baseline eGFR by Cockcroft-Gault of 7.2 mL/min (range, 4.8 to 12.0).
^d AUC_inf.
^e AUC_last.
^f AUC_tau.

Paediatric population

Steady-state pharmacokinetics of tenofovir alafenamide and its metabolite tenofovir were evaluated in HBV-infected paediatric patients 12 to <18 years weighing ≥35 kg and 6 to <12 years weighing ≥25 kg (Table 2).

Table 2. Pharmacokinetics of Tenofovir Alafenamide and its Metabolite Tenofovir in Paediatric Patients Aged 6 to <18 Years and Adults:

CV = coefficient of variation; TAF= tenofovir alafenamide; NA = not applicable
^a Population PK-derived parameters from Study 1092 (6 to <12 years old weighing ≥25 kg, N=12; 12 to <18 years old weighing ≥35 kg, N=47).
^b Population PK-derived parameters from Studies 108 and 110 (TAF: N=698, Tenofovir: N=856).

Non-clinical studies in rats and dogs revealed bone and kidney as the primary target organs of toxicity. Bone toxicity was observed as reduced BMD in rats and dogs at tenofovir exposures at least four times greater than those expected after administration of tenofovir alafenamide. A minimal infiltration of histiocytes was present in the eye in dogs at tenofovir alafenamide and tenofovir exposures of approximately 4 and 17 times greater, respectively, than those expected after administration of tenofovir alafenamide.

Tenofovir alafenamide was not mutagenic or clastogenic in conventional genotoxic assays.

Because there is a lower tenofovir exposure in rats and mice after tenofovir alafenamide administration compared to tenofovir disoproxil, carcinogenicity studies and a rat peri-postnatal study were conducted only with tenofovir disoproxil. No special hazard for humans was revealed in conventional studies of carcinogenic potential with tenofovir disoproxil (as fumarate) and toxicity to reproduction and development with tenofovir disoproxil (as fumarate) or tenofovir alafenamide. Reproductive toxicity studies in rats and rabbits showed no effects on mating, fertility, pregnancy or foetal parameters. However, tenofovir disoproxil reduced the viability index and weight of pups in a peri-postnatal toxicity study at maternally toxic doses. A long-term oral carcinogenicity study in mice showed a low incidence of duodenal tumours, considered likely related to high local concentrations in the gastrointestinal tract at the high dose of 600 mg/kg/day. The mechanism of tumour formation in mice and potential relevance for humans is uncertain.