Erlotinib is metabolized predominantly by CYP3A4, and inhibitors of CYP3A4 would be expected to increase exposure. Co-treatment with the potent CYP3A4 inhibitor ketoconazole increases erlotinib AUC by 2/3. When TARCEVA was co-administered with ciprofloxacin, an inhibitor of both CYP3A4 and CYP1A2, the erlotinib exposure [AUC] and maximum concentration [Cmax] increased by 39% and 17% respectively. Caution should be used when administering or taking TARCEVA with ketoconazole and other strong CYP3A4 inhibitors such as, but not limited to, atazanavir, clarithromycin, indinavir, itraconazole, nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin, troleandomycin (TAO), voriconazole and grapefruit or grapefruit juice. [see Dosage and Administration (2.3 )].

Pre-treatment with the CYP3A4 inducer rifampicin for 7 days prior to TARCEVA decreased erlotinib AUC by about 2/3 to 4/5, which is equivalent to a dose of about 30 to 50 mg in NSCLC patients. In a separate study, treatment with rifampicin for 11 days, with co-administration of a single 450 mg dose of TARCEVA on day 8 resulted in a mean erlotinib exposure (AUC) that was 57.6% of that observed following a single 150 mg TARCEVA dose in the absence of rifampicin treatment [see Dose Modifications (2.3 )]. Use of alternative treatments lacking CYP3A4 inducing activity is strongly recommended. If an alternative treatment is unavailable, adjusting the starting dose should be considered. If the TARCEVA dose is adjusted upward, the dose will need to be reduced immediately to the indicated starting dose upon discontinuation of rifampicin or other inducers. Other CYP3A4 inducers include, but are not limited to, rifabutin, rifapentine, phenytoin, carbamazepine, phenobarbital and St. John's Wort [see Dosage and Administration (2.3 )].

Cigarette smoking has been shown to reduce erlotinib AUC. Patients should be advised to stop smoking; however, if they continue to smoke, a cautious increase in the dose of TARCEVA may be considered, while monitoring the patient’s safety. If the TARCEVA dose is adjusted upward, the dose should be reduced immediately to the indicated starting dose upon cessation of smoking. [see Dosage and Administration(2.3 ) and Clinical Pharmacology(12.3)].

Pretreatment and co-administration of TARCEVA decreased the AUC of CYP3A4 substrate, midazolam, by 24%. The mechanism is not clear.

In a study, there were no significant effects of gemcitabine on the pharmacokinetics of erlotinib nor were there significant effects of erlotinib on the pharmacokinetics of gemcitabine.

Drugs that alter the pH of the upper GI tract may alter the solubility of erlotinib and reduce its bioavailability. Co-administration of TARCEVA with omeprazole, a proton pump inhibitor, decreased the erlotinib AUC by 46%. Increasing the dose of TARCEVA when co-administered with such agents is not likely to compensate for the loss of exposure. Since proton pump inhibitors affect pH of the upper GI tract for an extended period, separation of doses may not eliminate the interaction. The concomitant use of proton pump inhibitors with TARCEVA should be avoided if possible. The use of antacids may be considered in place of histamine 2 receptor blockers (H2 blockers) or proton pump inhibitors in patients receiving TARCEVA. However, no clinical study has been conducted to evaluate the effect of antacids on erlotinib pharmacokinetics. If an antacid is necessary, the antacid dose and the TARCEVA dose should be separated by several hours [see Clinical Pharmacology (12.3)].