P-glycoprotein and breast cancer resistance protein restrict the brain penetration of the CDK4/6 inhibitor palbociclib.

INTRODUCTION: Palbociclib is a cyclin dependent kinase (CDK) 4/6 inhibitor with nanomolar potency and was recently approved for treatment of breast cancer. The drug may also be useful in glioblastoma (GBM) and diffuse intrinsic pontine gliomas (DIPG), which often have an activated CDK4/6-retinoblastoma signaling pathway. However, GBM and DIPG spread widely into the surrounding brain, which calls for a CDK4/6 inhibitor with sufficient blood-brain barrier penetration. METHODS: We first performed in vitro transwell assays and demonstrate that palbociclib is a substrate of both P-gp and BCRP. Next, we conducted pharmacokinetic studies using wildtype, Abcg2(-/-), Abcb1a/b(-/-) and Abcg2; Abcb1a/b(-/-) mice. RESULTS: The plasma levels were about 3000 and 500 nM and similar in all genotypes at 1 and 4 h after i.v. administration of 10 mg/kg. At 4 h the brain-to-plasma ratios were 0.3 in WT and Abcg2(-/-) mice versus 5.5 and 15 in Abcb1a/b(-/-) and Abcg2; Abcb1a/b(-/-) mice, respectively. The oral bioavailability of palbociclib was high (63 %) in WT mice and increased only modestly and non-significantly in Abcg2; Abcb1a/b(-/-) mice. The plasma level after oral dosing of 150 mg/kg was already much higher than observed in patients (200-400 nM) and exceeded 2500 nM for up to 24 h. This latter dose is commonly used in preclinical studies, which calls into question their predictive value as they were conducted at dose levels causing a clinically non-relevant systemic drug exposure. CONCLUSION: Thus, the brain penetration of palbociclib is restricted by P-gp and BCRP, which may restrict the efficacy against GBM and DIPG. Moreover, preclinical studies with this agent should be conducted at a more clinically relevant dose level.