- Eligibility of patients receiving any medications or substances known to affect or with the potential to affect the activity or pharmacokinetics of erlotinib will be determined following review of their case by the principal investigator; although concomitant use of cytochrome P450, family 3, subfamily A, polypeptide 4 (CYP3A4) inducers is not prohibited in this study, identification of MTD and DLT may be affected by their use; concomitant use of any of these drugs will be noted in the case report forms and will be taken into account in determining MTD and DLT of this therapy; efforts should be made to switch patients with a history of brain metastases who are taking enzyme-inducing anticonvulsant agents to other medications