Clinical Pharmacokinetics of Tyrosine Kinase Inhibitors

Pyrimidine (imatinib, dasatinib, nilotinib and pazopanib), pyridine (sorafenib) and pyrrole (sunitinib) tyrosine kinase inhibitors (TKIs) are multi-targeted TKIs with high activity towards several families of receptor and non-receptor tyrosine kinases involved in angiogenesis, tumour growth and metastatic progression of cancer. These orally administered TKIs have quite diverse characteristics with regard to absorption from the gastrointestinal tract. Absolute bioavailability in humans has been investigated only for imatinib (almost 100%) and pazopanib (14–39%; n = 3). On the basis of human radioactivity data, dasatinib is considered to be well absorbed after oral administration (19% and 0.1% of the total radioactivity were excreted as unchanged dasatinib in the faeces and urine, respectively). Quite low absolute bioavailability under fasted conditions is assumed for nilotinib (31%), sorafenib (50%) and sunitinib (50%). Imatinib, dasatinib and sunitinib exhibit dose-proportional increases in their area under the plasma concentration-time curve values over their therapeutic dose ranges. Less than dose-proportional increases were observed for nilotinib at doses ≥400 mg/day and for sorafenib and pazopanib at doses ≥800 mg/day. At steady state, the accumulation ratios are 1.5–2.5 (unchanged imatinib), 2.0 (nilotinib once-daily dosing), 3.4 (nilotinib twice-daily dosing), 1.2–4.5 (pazopanib), 5.7–6.4 (sorafenib) and 3.0–4.5 (sunitinib). Concomitant intake of a high-fat meal does not alter exposure to imatinib, dasatinib and sunitinib but leads to considerably increased bioavailability of nilotinib and pazopanib and decreased bioavailability of sorafenib. With the exception of pazopanib, the TKIs described here have large apparent volumes of distribution, exceeding the volume of body water by at least 4-fold.

Very low penetration into the central nervous system in humans has been reported for imatinib and dasatinib, but there are currently no published human data for nilotinib, pazopanib, sorafenib or sunitinib. All TKIs that have been described are more than 90% bound to the plasma proteins: α₁-acid glycoprotein and/or albumin. They are metabolized primarily via cytochrome P450 (CYP) 3A4, the only exception being sorafenib, for which uridine diphosphate glucuronosyltransferase 1A9 is the other main enzyme involved. Active metabolites of imatinib and sunitinib contribute to their antitumour activity. Although some patient demographics have been identified as significant co-factors that partly explain interindividual variability in exposure to TKIs, these findings have not been regarded as sufficient to recommend age-, sex-, bodyweight-or ethnicity-specific dose adjustment. Systemic exposure to imatinib, sorafenib and pazopanib increases in patients with hepatic impairment, and reduction of the initial therapeutic dose is recommended in this subpopulation. The starting dose of imatinib should also be reduced in renally impaired subjects. Because the solubility of dasatinib is pH dependent, co-administration of histamine H₂-receptor antagonists and proton pump inhibitors with dasatinib should be avoided. With the exception of sorafenib, systemic exposure to TKIs is significantly decreased/increased by co-administration of potent CYP3A4 inducers/inhibitors, and so it is strongly recommended that the TKI dose is adjusted or that such co-administration is avoided. Caution is also recommended for co-administration of CYP3A4 substrates with TKIs, especially for those with a narrow therapeutic index. However, current recommendations with regard to dose adjustment of TKIs need to be validated in clinical studies. Further investigations are needed to explain the large interindividual variability in the pharmacokinetics of these drugs and to assess the clinical relevance of their interaction potential and inhibitory effects on metabolizing enzymes and transporters.