Central nervous system (CNS) relapse accompanying prolonged administration of imatinib mesylate, an Abl-specific tyrosine kinase inhibitor, has recently become apparent as an impediment to the therapy of Philadelphia-chromosome-positive (Ph+) leukemia. CNS relapse may be explained by limited penetration of imatinib into the cerebrospinal fluid due to presence of P-glycoprotein (P-gp) at blood-brain barrier. To overcome imatinib-resistance mechanisms such as bcr-abl gene amplification, point mutations within ABL kinase domain, and activation of Lyn, we recently developed a specific dual BCR-ABL/Lyn inhibitor, INNO-406 (formerly NS-187), which is 25–55 times more potent than imatinib in vitro and at least 10 times more potent in vivo (
Blood 106: 3948–3954, 2005
). The aim of this study was to investigate the efficacy of INNO-406 in treating CNS Ph+ leukemia. The intracellular accumulation of [14C]INNO-406 in P-gp overexpressing LLC-GA5-COL150 cells was much less than that in parental LLC-PK1 cells. The addition of 10 mM cyclosporin A (CsA) increased the intracellular accumulation of [14C]INNO-406 in both LLC-PK1 cells and LLC-GA5-COL150 cells. The peak concentration of INNO-406 in the brain when 30 mg/kg INNO-406 was administered p.o. was 50 ng/ g (87 nM), representing only 10% of plasma drug level. These findings suggested that INNO-406 is also a substrate of P-gp, as is imatinib. However, the residual concentration of INNO-406 in the CNS was enough to inhibit the growth of Ph+ leukemic cells according to the in vitro data. To increase the concentration of INNO-406 in CNS, we next examined the combined effects of CsA. In the brain, the concentration of INNO-406 was doubled following prior administration of 50 mg/kg CsA. Since pharmacokinetic studies suggested the possible effects of INNO-406 against CNS Ph+ leukemia, we investigated in vivo anti-CNS Ph+ leukemia effects of INNO-406 alone and combination of INNO-406 and CsA using immunodeficient mice (nude or NOD/SCID) which received Ph+ leukemic cells into the cerebral ventricle. INNO-406 alone inhibited growth of leukemic cells harboring either wild type or mutated BCR-ABL such as E255K and M351T in CNS. Furthermore, CsA significantly enhanced anti-CNS Ph+ leukemia effects of INNO-406 in vivo not only against cells harboring wild type BCR-ABL but also against cells harboring BCR-ABL/M351T (Figure). In conclusion, INNO-406 was found to inhibit Ph+ leukemic cell growth in CNS in spite of efflux of the compound by P-gp, and CsA augmented the anti-CNS Ph+ leukemia effects of INNO-406. Phase I clinical study on INNO-406 was initiated in the U.S.A. in July 2006. The efficacy and safety of INNO-406 in the treatment of leukemias is expected to be verified by early-phase clinical trials.
Disclosures: This work was partly supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
