Before turning back to brain metastases, I wanted to cover a topic that has generated some recent questions, and that is the issue of potential interactions of tarceva with food and other drugs. Just as an introduction, the standard dose of single-agent tarceva in lung cancer is 150 mg by mouth daily, and this is meant to be taken on an empty stomach, at least one-hour before or two hours after eating. Taking tarceva with food can make the absorption of tarceva greater but is overall so variable that it's very hard to know what kind of blood levels to expect, so the established target is 150 mg taken on an empty stomach. Tarceva is broken down by a collection of liver enzymes known as the CYP (pronounced "sip") family, and particularly a liver enzyme known as CYP 3A4. These enzymes can also be found in the intestinal lining and can affect absorption of drugs like Tarceva. The CYP enzymes, and particularly CYP 3A4, can have their levels affected by other medicines and foods. Interestingly, tobacco smoking is another factor that can increase clearance of tarceva: in active smokers, tarceva clearance is about 25% faster, so the blood levels are lower. The potential that tarceva may need a different dose level for active (not likely former) smokers is a question that needs to be addressed further, and one I'll have to cover in a separate post. The medicines that inhibit CYP 3A4 can dramatically increase levels of tarceva. These are drugs like ketoconazole, an antifungal medicine, and other antifungal and antiviral drugs, including several used to treat HIV/AIDS. Grapefruit juice also inhibits CYP3A4. Any of these agents can decrease tarceva clearance by about 2/3, so reducing the dose of tarceva should be considered, particularly if a patient is experiencing significant tarceva-related side effects. On the flipside, a bunch of other medicines can significantly increase CYP3A4 activity and reduce tarceva levels by about 2/3. These include an antibiotic called rifampin, multiple anti-epilepsy drugs (including dilantin, tegretol, and phenobarbital), and St. John's Wort. One recommendation in the tarceva product information is to not take these other drugs if there is a possible alternative, but otherwise, it is recommended that a higher dose than 150 mg daily be considered. In fact, studies of patients with primary brain tumors, who often need to be on anti-convulsant drugs like dilantin, have demontrasted that higher doses of 300 mg or even up to 500 mg daily are safe (abstract here), and that the 500 mg daily dose in someone on a CYP 3A4 inducing-drug like dilantin is associated with tarceva blood levels similar to the 150 mg dose in someone who isn't on one of these CYP 3A4 inducers (abstract here). While there isn't an official recommendation to prescribe a higher dose, the FDA acknowledges that this is something that should be considered for patients on drugs that are known to increase tarceva clearance.

Another potentially important interaction can occur between tarceva and the oral blood-thinner coumadin, which many patients need to be on because of a history of blood clots or atrial fibrillation. There is a tendency for patients to run higher International Normalized Ratio (INR) results, in other words to have blood be too thinned, when tarceva is added, so it is recommended that patients on coumadin have their INR checked frequently, particularly in the first few weeks, when a new equilibrium is being established. This certainly doesn't cover every drug that can interact with tarceva, but those are the highlights. There's more detail in the official document of tarceva product information.