Lapatinib

Lapatinib, a 4-anilinoquinazoline, is an inhibitor of the intracellular tyrosine kinase domains of both EGFR (ErbB1) and of HER2 (ErbB2) receptors (estimated Ki^app values of 3nM and 13nM, respectively) with a slow off-rate from these receptors (half-life greater than or equal to 300 minutes). Lapatinib inhibits ErbB-driven tumour cell growth in vitro and in various animal models.

The combination of lapatinib and trastuzumab may offer complementary mechanisms of action as well as possible non-overlapping mechanisms of resistance. The growth inhibitory effects of lapatinib were evaluated in trastuzumab-conditioned cell lines. Lapatinib retained significant activity against HER2-amplified breast cancer cell lines selected for long-term growth in trastuzumab-containing medium in vitro and was synergistic in combination with trastuzumab in these cell lines.

Absorption

The absolute bioavailability following oral administration of lapatinib is unknown, but it is incomplete and variable (approximately 70% coefficient of variation in AUC). Serum concentrations appear after a median lag time of 0.25 hours (range 0 to 1.5 hours). Peak plasma concentrations (C_max) of lapatinib are achieved approximately 4 hours after administration. Daily dosing of 1250 mg produces steady state geometric mean (coefficient of variation) C_max values of 2.43 (76%) μg/ml and AUC values of 36.2 (79%) μg*hr/ml.

Systemic exposure to lapatinib is increased when administered with food. Lapatinib AUC values were approximately 3- and 4-fold higher (C_max approximately 2.5 and 3–fold higher) when administered with a low fat (5% fat [500 calories]) or with a high fat (50% fat [1,000 calories]) meal, respectively, as compared with administration in the fasted state. Systemic exposure to lapatinib is also affected by the timing of administration in relation to food intake. Relative to dosing 1 hour before a low fat breakfast, mean AUC values were approximately 2- and 3-fold higher when lapatinib was administered 1 hour after a low fat or high fat meal, respectively.

Distribution

Lapatinib is highly bound (greater than 99%) to albumin and alpha-1 acid glycoprotein. In vitro studies indicate that lapatinib is a substrate for the transporters BCRP (ABCG1) and p-glycoprotein (ABCB1). Lapatinib has also been shown in vitro to inhibit these efflux transporters, as well as the hepatic uptake transporter OATP 1B1, at clinically relevant concentrations (IC₅₀ values were equal to 2.3 μg/ml). The clinical significance of these effects on the pharmacokinetics of other medicinal products or the pharmacological activity of other anti-cancer medicinal products is not known.

Biotransformation

Lapatinib undergoes extensive metabolism, primarily by CYP3A4 and CYP3A5, with minor contributions from CYP2C19 and CYP2C8 to a variety of oxidated metabolites, none of which account for more than 14% of the dose recovered in the faeces or 10% of lapatinib concentration in plasma. Lapatinib inhibits CYP3A (Ki 0.6 to 2.3 μg/ml) and CYP2C8 (0.3 μg/ml) in vitro at clinically relevant concentrations. Lapatinib did not significantly inhibit the following enzymes in human liver microsomes: CYP1A2, CYP2C9, CYP2C19, and CYP2D6 or UGT enzymes (in vitro IC₅₀ values were greater than or equal to 6.9 μg/ml).

Elimination

The half-life of lapatinib measured after single doses increases with increasing dose. However, daily dosing of lapatinib results in achievement of steady state within 6 to 7 days, indicating an effective half-life of 24 hours. Lapatinib is predominantly eliminated through metabolism by CYP3A4/5. Biliary excretion may also contribute to the elimination. The primary route of excretion for lapatinib and its metabolites is in faeces. Recovery of unchanged lapatinib in faeces accounts for a median 27% (range 3 to 67%) of an oral dose. Less than 2% of the administered oral dose (as lapatinib and metabolites) excreted in urine.

Renal impairment

Lapatinib pharmacokinetics have not been specifically studied in patients with renal impairment or in patients undergoing haemodialysis. Available data suggest that no dose adjustment is necessary in patients with mild to moderate renal impairment.

Hepatic impairment

The pharmacokinetics of lapatinib were examined in patients with moderate (n=8) or severe (n=4) hepatic impairment (Child-Pugh scores of 7-9, or greater than 9, respectively) and in 8 healthy control patients. Systemic exposure (AUC) to lapatinib after a single oral 100 mg dose increased approximately 56% and 85% in patients with moderate and severe hepatic impairment, respectively. Administration of lapatinib in patients with hepatic impairment should be undertaken with caution.

Lapatinib was studied in pregnant rats and rabbits given oral doses of 30, 60, and 120 mg/kg/day. There were no teratogenic effects; however, minor anomalies (left-sided umbilical artery, cervical rib and precocious ossification) occurred in rats at ≥60 mg/kg/day (4 times the expected human clinical exposure). In rabbits, lapatinib was associated with maternal toxicity at 60 and 120 mg/kg/day (8% and 23% of the expected human clinical exposure, respectively) and abortions at 120 mg/kg/day. At ≥60 mg/kg/day there were decreased foetal body weights, and minor skeletal variations. In the rat pre- and postnatal development study, a decrease in pup survival occurred between birth and postnatal day 21 at doses of 60 mg/kg/day or higher (5 times the expected human clinical exposure). The highest no-effect dose for this study was 20 mg/kg/day.

In oral carcinogenicity studies with lapatinib, severe skin lesions were seen at the highest doses tested which produced exposures based on AUC up to 2-fold in mice and male rats, and up to 15-fold in female rats, compared to humans given 1250 mg of lapatinib once daily. There was no evidence of carcinogenicity in mice. In rats, the incidence of benign haemangioma of the mesenteric lymph nodes was higher in some groups than in concurrent controls. There was also an increase in renal infarcts and papillary necrosis in female rats at exposures 7 and 10-fold compared to humans given 1250 mg of lapatinib once daily. The relevance of these findings for humans is uncertain.

There were no effects on male or female rat gonadal function, mating, or fertility at doses up to 120 mg/kg/day (females) and up to 180 mg/kg/day (males) (8 and 3 times the expected human clinical exposure, respectively). The effect on human fertility is unknown.

Lapatinib was not clastogenic or mutagenic in a battery of assays including the Chinese hamster chromosome aberration assay, the Ames assay, human lymphocyte chromosome aberration assay and an in vivo rat bone marrow chromosome aberration assay.