Lapatinib ditosylate

7 DRUG INTERACTIONS TYKERB is likely to increase exposure to concomitantly administered drugs which are substrates of CYP3A4, CYP2C8, or P-glycoprotein (ABCB1). ( 7.1 ) Avoid strong CYP3A4 inhibitors. If unavoidable, consider dose reduction of TYKERB in patients coadministered a strong CYP3A4 inhibitor. ( 2.2 , 7.2 ) Avoid strong CYP3A4 inducers. If unavoidable, consider gradual dose increase of TYKERB in patients coadministered a strong CYP3A4 inducer. ( 2.2 , 7.2 )

7.1 Effects of Lapatinib on Drug-Metabolizing Enzymes and Drug Transport Systems Lapatinib inhibits CYP3A4, CYP2C8, and P-glycoprotein (P-gp, ABCB1) in vitro at clinically relevant concentrations and is a weak inhibitor of CYP3A4 in vivo. Caution should be exercised and dose reduction of the concomitant substrate drug should be considered when dosing TYKERB concurrently with medications with narrow therapeutic windows that are substrates of CYP3A4, CYP2C8, or P-gp. Lapatinib did not significantly inhibit the following enzymes in human liver microsomes: CYP1A2, CYP2C9, CYP2C19, and CYP2D6 or UGT enzymes in vitro, however, the clinical significance is unknown.

Midazolam : Following coadministration of TYKERB and midazolam (CYP3A4 substrate), 24-hour systemic exposure (AUC) of orally administered midazolam increased 45%, while 24-hour AUC of intravenously administered midazolam increased 22%. Paclitaxel : In cancer patients receiving TYKERB and paclitaxel (CYP2C8 and P-gp substrate), 24-hour systemic exposure (AUC) of paclitaxel was increased 23%. This increase in paclitaxel exposure may have been underestimated from the in vivo evaluation due to study design limitations. Digoxin : Following coadministration of TYKERB and digoxin (P-gp substrate), systemic AUC of an oral digoxin dose increased approximately 2.8-fold.

Serum digoxin concentrations should be monitored prior to initiation of TYKERB and throughout coadministration. If digoxin serum concentration is greater than 1.2 ng/mL, the digoxin dose should be reduced by half.

7.2 Drugs That Inhibit or Induce Cytochrome P450 3A4 Enzymes Lapatinib undergoes extensive metabolism by CYP3A4, and concomitant administration of strong inhibitors or inducers of CYP3A4 alter lapatinib concentrations significantly (see Ketoconazole and Carbamazepine sections, below) . Dose adjustment of TYKERB should be considered for patients who must receive concomitant strong inhibitors or concomitant strong inducers of CYP3A4 enzymes [see Dosage and Administration (2.2)] .

Ketoconazole : In healthy subjects receiving ketoconazole, a CYP3A4 inhibitor, at 200 mg twice daily for 7 days, systemic exposure (AUC) to lapatinib was increased to approximately 3.6-fold of control and half-life increased to 1.7-fold of control. Carbamazepine : In healthy subjects receiving the CYP3A4 inducer, carbamazepine, at 100 mg twice daily for 3 days and 200 mg twice daily for 17 days, systemic exposure (AUC) to lapatinib was decreased approximately 72%.

7.3 Drugs That Inhibit Drug Transport Systems Lapatinib is a substrate of the efflux transporter P-glycoprotein (P-gp, ABCB1). If TYKERB is administered with drugs that inhibit P-gp, increased concentrations of lapatinib are likely, and caution should be exercised.

7.4 Acid-Reducing Agents The aqueous solubility of lapatinib is pH dependent, with higher pH resulting in lower solubility. However, esomeprazole, a proton pump inhibitor, administered at a dose of 40 mg once daily for 7 days, did not result in a clinically meaningful reduction in lapatinib steady-state exposure.

8.1 Pregnancy Risk Summary Based on findings in animal studies and its mechanism of action, TYKERB can cause fetal harm when administered to a pregnant woman [see Clinical Pharmacology (12.1)] . There are no available human data to inform of the drug-associated risks. In an animal reproduction study, administration of lapatinib to pregnant rats during organogenesis and through lactation led to death of offspring within the first 4 days after birth at maternal exposures that were ≥ 3.3 times the human clinical exposure based on AUC following 1,250 mg dose of lapatinib plus capecitabine.

When administered to pregnant animals during the period of organogenesis, lapatinib caused fetal anomalies (rats) or abortions (rabbits) at maternally toxic doses (see Data) . Advise pregnant women and females of reproductive potential of the potential risk to the fetus. The background risk of major birth defects and miscarriage for the indicated population is unknown; however, in the U.S. general population, the estimated background risk of major birth defects is 2% to 4% and of miscarriage is 15% to 20% of clinically recognized pregnancies.

Data Animal Data In embryo-fetal development studies in rats and rabbits, pregnant animals received oral doses of lapatinib at 30, 60, and 120 mg/kg/day during the period of organogenesis. Minor anomalies (left-sided umbilical artery, cervical rib, and precocious ossification) occurred in rats at the maternally toxic dose of 120 mg/kg/day (approximately 6.4 times the human clinical exposure based on AUC following 1,250 mg dose of lapatinib plus capecitabine).

In rabbits, lapatinib was associated with maternal toxicity at 60 and 120 mg/kg/day (approximately 0.07 and 0.2 times the human clinical exposure, respectively, based on AUC following 1,250 mg dose of lapatinib plus capecitabine) and abortions at 120 mg/kg/day. Maternal toxicity was associated with decreased fetal body weights and minor skeletal variations. In a pre- and post-natal development study, rats were given oral doses of 20, 60, and 120 mg/kg/day during gestation through lactation up to weaning.

In rats, doses of 60 and 120 mg/kg/day (approximately 3.3 and 6.4 times the human clinical exposure, respectively, based on AUC following 1,250 mg dose of lapatinib plus capecitabine) led to decrease in F1 postnatal survival (91% and 34% of the pups died by the fourth day after birth, at 60 and 120 mg/kg/day, respectively).