Pazopanib

Pazopanib is an orally administered, potent multi-target tyrosine kinase inhibitor (TKI) of vascular endothelial growth factor receptors (VEGFR) -1, -2, and -3, platelet-derived growth factor (PDGFR) -α and -β, and stem cell factor receptor (c-KIT), with IC₅₀ values of 10, 30, 47, 71, 84 and 74 nM, respectively.

In preclinical experiments, pazopanib dose-dependently inhibited ligand-induced auto-phosphorylation of VEGFR-2, c-Kit and PDGFR-receptors in cells. In vivo, pazopanib inhibited VEGF-induced VEGFR-2 phosphorylation in mouse lungs, angiogenesis in various animal models, and the growth of multiple human tumour xenografts in mice.

Pharmacogenomics

In a pharmacogenetic meta-analysis of data from 31 clinical studies of pazopanib administered either as monotherapy or in combination with other agents, ALT >5 x ULN (NCI CTC Grade 3) occurred in 19% of HLA-B*57:01 allele carriers and in 10% of non-carriers. In this dataset, 133/2235 (6%) of the patients carried the HLA-B*57:01 allele.

Absorption

Upon oral administration of a single pazopanib 800 mg dose to patients with solid tumours, maximum plasma concentration (C_max) of approximately 19 ± 13 μg/ml was obtained after median 3.5 hours (range 1.0-11.9 hours) and an AUC_0-∞ of approximately 650 ± 500 μg.h/ml was obtained. Daily dosing results in 1.23- to 4-fold increase in AUC_0-T.

There was no consistent increase in AUC or C_max at pazopanib doses above 800 mg.

Systemic exposure to pazopanib is increased when administered with food. Administration of pazopanib with a high-fat or low-fat meal results in an approximately 2-fold increase in AUC and C_max. Therefore, pazopanib should be administered at least two hours after food or at least one hour before food.

Administration of a pazopanib 400 mg crushed tablet increased AUC_(0-72) by 46% and C_max by approximately 2 fold and decreased t_max by approximately 2 hours compared to administration of the whole tablet. These results indicate that the bioavailability and the rate of pazopanib oral absorption are increased after administration of the crushed tablet relative to administration of the whole tablet.

Distribution

Binding of pazopanib to human plasma protein in vivo was greater than 99% with no concentration dependence over the range of 10-100 μg/ml. In vitro studies suggest that pazopanib is a substrate for P-gp and BCRP.

Biotransformation

Results from in vitro studies demonstrated that metabolism of pazopanib is mediated primarily by CYP3A4, with minor contributions from CYP1A2 and CYP2C8. The four principle pazopanib metabolites account for only 6% of the exposure in plasma. One of these metabolites inhibits the proliferation of VEGF-stimulated human umbilical vein endothelial cells with a similar potency to that of pazopanib, the others are 10- to 20-fold less active. Therefore, activity of pazopanib is mainly dependent on parent pazopanib exposure.

Elimination

Pazopanib is eliminated slowly with a mean half-life of 30.9 hours after administration of the recommended dose of 800 mg. Elimination is primarily via faeces with renal elimination accounting for <4% of the administered dose.

Special populations

Renal impairment

Results indicate that less than 4% of an orally administered pazopanib dose is excreted in the urine as pazopanib and metabolites. Results from population pharmacokinetic modelling (data from subjects with baseline CL_CR values ranging from 30.8 ml/min to 150 ml/min) indicated that renal impairment is unlikely to have clinically relevant effect on pazopanib pharmacokinetics. No dose adjustment is required in patients with creatinine clearance above 30 ml/min. Caution is advised in patients with creatinine clearance below 30 ml/min as there is no experience of pazopanib in this patient population.

Hepatic impairment

Mild:

The median steady-state pazopanib C_max and AUC_(0-24) in patients with mild abnormalities in hepatic parameters (defined as either normal bilirubin and any degree of ALT elevation or as an elevation of bilirubin up to 1.5 x ULN regardless of the ALT value) after administration of 800 mg once daily are similar to the median in patients with normal hepatic function (see Table 7). 800 mg pazopanib once daily is the recommended dose in patients with mild abnormalities of serum liver tests.

Moderate:

The maximally tolerated pazopanib dose (MTD) in patients with moderate hepatic impairment (defined as an elevation of bilirubin >1.5 x to 3 x ULN regardless of the ALT values) was 200 mg once daily. The median steady-state C_max and AUC_(0-24) values after administration of 200 mg pazopanib once daily in patients with moderate hepatic impairment were approximately 44% and 39%, of the corresponding median values after administration of 800 mg once daily in patients with normal hepatic function, respectively (see Table 7).

Based on safety and tolerability data, the dose of pazopanib should be reduced to 200 mg once daily in subjects with moderate hepatic impairment.

Severe:

The median steady-state C_max and AUC_(0-24) values after administration of 200 mg pazopanib once daily in patients with severe hepatic impairment were approximately 18% and 15%, of the corresponding median values after administration of 800 mg once daily in patients with normal hepatic function. Based on the diminished exposure and limited hepatic reserve pazopanib is not recommended in patients with severe hepatic impairment (defined as total bilirubin >3 X ULN regardless of any level of ALT).

Table 7. Median steady-state pazopanib pharmacokinetics measured in subjects with hepatic impairment:

OD – once daily

The preclinical safety profile of pazopanib was assessed in mice, rats, rabbits and monkeys. In repeat dose studies in rodents, effects in a variety of tissues (bone, teeth, nail beds, reproductive organs, haematological tissues, kidney and pancreas) appear related to the pharmacology of VEGFR inhibition and/or disruption of VEGF signalling pathways, with most effects occurring at plasma exposure levels below those observed in the clinic. Other observed effects include body weight loss, diarrhoea and/or morbidity that were either secondary to local gastrointestinal effects caused by high local mucosal medicinal product exposure (monkeys) or pharmacological effects (rodents). Proliferative hepatic lesions (eosinophilic foci and adenoma) were seen in female mice at exposures 2.5 times human exposure based on AUC.

In juvenile toxicity studies, when pre-weaning rats were dosed from day 9 post partum through to day 14 post partum, pazopanib caused mortalities and abnormal organ growth/maturation in kidney, lung, liver and heart, at a dose approximately 0.1 times the clinical exposure based on AUC in adult humans. When post-weaning rats were dosed from day 21 post partum to day 62 post partum, toxicological findings were similar to adult rats at comparable exposures. Human paediatric patients are at increased risk for bone and teeth effects as compared to adults, as these changes, including inhibition of growth (shortened limbs), fragile bones and remodelling of teeth, were present in juvenile rats at ≥10 mg/kg/day (equal to approximately 0.1-0.2 times the clinical exposure based on AUC in adult humans).

Reproductive, fertility and teratogenic effects

Pazopanib has been shown to be embryotoxic and teratogenic when administered to rats and rabbits at exposures more than 300-fold lower than the human exposure (based on AUC). Effects included reduced female fertility, increased pre- and post-implantation loss, early resorptions, embryo lethality, decreased foetal body weight and cardiovascular malformation. Decreased corpora lutea, increased cysts and ovarian atrophy have also been noted in rodents. In a rat male fertility study, there was no effect on mating or fertility, but decreased testicular and epididymal weights were noted with reductions in sperm production rates, sperm motility, and epididymal and testicular sperm concentrations observed at exposures 0.3 times human exposure based on AUC.

Genotoxicity

Pazopanib did not cause genetic damage when tested in genotoxicity assays (Ames assay, human peripheral lymphocyte chromosome aberration assay and rat in vivo micronucleus). A synthetic intermediate in manufacture of pazopanib, which is also present in the final drug substance in low amounts, was not mutagenic in the Ames assay but genotoxic in the mouse lymphoma assay and in vivo mouse micronucleus assay.

Carcinogenicity

In two-year carcinogenicity studies with pazopanib, there were increased numbers of liver adenomas noted in mice and duodenal adenocarcinomas noted in rats. Based on the rodent-specific pathogenesis and mechanism for these findings, they are not considered to represent an increased carcinogenic risk for patients taking pazopanib.