Lapatinib

BRAND NAMES

• Tykerb (United States and several other countries) 
• Tyverb (European Union and many international markets)

MECHANISM OF ACTION

Lapatinib is a dual tyrosine kinase inhibitor that blocks the intracellular signaling of HER2 and EGFR receptors. By reversibly inhibiting their kinase activity, it prevents activation of downstream pathways involved in cell growth and survival, such as PI3K/AKT and RAS/RAF/MEK. This leads to reduced tumor cell proliferation and increased cancer cell death, especially in HER2-positive breast cancer.

PHARMACOKINETICS

Absorption

Lapatinib is orally absorbed, but its bioavailability is variable and incomplete. Absorption is significantly increased when taken with food, especially high-fat meals, which can raise plasma concentrations several-fold. Peak plasma levels are typically reached within about 3 to 6 hours after oral administration. The drug is highly protein-bound and undergoes extensive first-pass metabolism, mainly via the CYP3A4 enzyme system, which also contributes to variability in systemic exposure.

Distribution

Lapatinib has a large volume of distribution and is extensively distributed into tissues. It is highly protein bound (>99%), mainly to albumin, which leaves only a small free fraction in plasma. Its penetration into tumors is adequate, but CNS distribution is limited due to efflux transporters like P-glycoprotein.

Metabolism

Lapatinib is primarily metabolized in the liver by the CYP3A4 enzyme system into inactive metabolites. Because of this, its levels can be affected by drugs that inhibit or induce CYP3A4, leading to increased toxicity or reduced efficacy respectively.

Elimination

Lapatinib is eliminated mainly through hepatic metabolism, with most of the drug excreted in feces as metabolites.  It has a relatively long elimination half-life, contributing to once-daily dosing in clinical use.

PHARMACODYNAMICS

Lapatinib exerts its pharmacodynamic effects by selectively and reversibly inhibiting the tyrosine kinase activity of HER2 and EGFR receptors. This blocks downstream signaling pathways such as PI3K/AKT and RAS/RAF/MEK/ERK, which are responsible for tumor cell proliferation and survival. The result is reduced growth of HER2-positive cancer cells and induction of apoptosis, with therapeutic effects that are enhanced when combined with other anticancer agents like capecitabine.

ADMINISTRATION

Lapatinib is taken orally, usually once daily. It should be administered on an empty stomach, at least one hour before or after food, because food can significantly increase its absorption. The dosing schedule may vary depending on whether it is used alone or in combination therapy for HER2-positive breast cancer.

DOSAGE AND STRENGTH

Lapatinib is commonly available as tablet formulations of 250 mg strength. The usual adult dose varies by indication: in HER2-positive metastatic breast cancer it is often 1250 mg once daily when combined with capecitabine, and 1500 mg once daily when used with letrozole in hormone receptor–positive disease. Dose adjustments may be required based on toxicity, liver function, or drug interactions, particularly with CYP3A4 inhibitors or inducers.

DRUG INTERACTIONS

Lapatinib has significant drug interactions mainly due to its metabolism via CYP3A4. Strong CYP3A4 inhibitors (such as ketoconazole, ritonavir, and clarithromycin) can increase lapatinib levels and risk of toxicity, while CYP3A4 inducers (such as rifampin, carbamazepine, and phenytoin) can reduce its effectiveness. It may also interact with drugs that prolong QT interval or affect hepatic function, requiring careful monitoring and possible dose adjustments.

FOOD INTERACTIONS

Lapatinib should be taken on an empty stomach because food, especially high-fat meals, can significantly increase its absorption and drug levels. Patients are advised to take it at least 1 hour before or 1 hour after meals. Grapefruit products should also be avoided as they can increase exposure by inhibiting CYP3A4 metabolism.

CONTRAINDICATIONS

Lapatinib is contraindicated in patients with known hypersensitivity to the drug or any of its components. It should be avoided in individuals with severe hepatic impairment due to its extensive liver metabolism and risk of toxicity. Caution is also required in patients with conditions predisposing to QT prolongation or in those taking strong CYP3A4 inhibitors or inducers, as these can significantly alter drug levels and safety.

SIDE EFFECTS

• Diarrhea

• Rash

• Nausea

• Vomiting

• Fatigue

• Decreased appetite

• Hepatotoxicity (↑ liver enzymes)

• Left ventricular dysfunction

• Interstitial lung disease (rare)

OVER DOSE

• Severe diarrhea

• Vomiting

• Dehydration

• Rash

• Fatigue

• Hepatotoxicity (↑ liver enzymes)

• Left ventricular dysfunction

• Electrolyte imbalance

• Supportive treatment required (no antidote)

TOXICITY

Lapatinib toxicity mainly involves hepatotoxicity, which may present as elevated liver enzymes or liver injury, along with gastrointestinal effects like severe diarrhea, nausea, and vomiting. Dermatologic reactions such as rash and systemic effects like fatigue are also common. More serious toxicities include left ventricular dysfunction, rare QT prolongation, and occasional interstitial lung disease, requiring close clinical monitoring during therapy.