Clinical Pharmacokinetics of Tyrosine Kinase Inhibitors

The 4-anilinoquinazolines (gefitinib, erlotinib and lapatinib) are members of a class of potent and selective inhibitors of the human epidermal growth factor receptor (HER) family of tyrosine kinases that have been developed to treat patients with tumours with defined genetic alterations of the HER tyrosine kinase domain. They are characterized by a moderate rate of absorption after oral administration with peak plasma concentrations at several hours post-dose. Absolute bioavailability of gefitinib and erlotinib is about 60%. Low bioavailability is assumed for lapatinib. The drugs are extensively distributed in human tissues, including tumour tissues, have a large volume of distribution at least 3-fold exceeding the volume of body water and are extensively (about 95%) protein bound to α1-acid glycoprotein and albumin.Existing human data for gefitinib and erlotinib indicate that these substances penetrate into the central nervous system and accumulate in brain tumours, possibly due to leaks in the blood-brain barrier. Gefitinib, erlotinib and the absorbed fraction of lapatinib undergo extensive metabolism — mainly via hepatic and intestinal cytochrome P450 (CYP) 3A4 and also via CYP2D6 (gefitinib) and CYP1A2 (erlotinib) — and are primarily eliminated by biotransformation. The excretion of unchanged gefitinib, erlotinib, lapatinib and their metabolites occurs predominantly in the faeces and only a minor fraction is excreted in the urine. No relevant effects of age, sex, bodyweight or race on their pharmacokinetics have been reported to date.Limited available data indicate that genetic polymorphisms in enzymes and transporters involved in the pharmacokinetics of gefitinib (CYP2D6) and erlotinib (CYP3A4, CYP3A5 and ABCG2 [breast cancer resistance protein]) alter the exposure to these drugs. Modification of drug dose should be considered in patients with severe hepatic impairment receiving these tyrosine kinase inhibitors and in current smokers receiving erlotinib. Existing recommendations for dose adjustment (i.e. a dose decrement or increment for gefitinib, erlotinib and lapatinib in the presence of CYP3A4 inhibitors or inducers, respectively; a dose increase for erlotinib in smoking patients) need to be validated in clinical studies. Further investigations are required to explain the large interindividual variability in the pharmacokinetics of these drugs and to assess the clinical relevance of interaction potential and inhibitory effects on the metabolizing enzymes and transporters.