MIAMI BEACH -The Bcr- Abl tyrosine kinase inhibitor PD173955 (PD17) binds to the target ATP binding pocket even more efficiently than STI-571 (imatinib mesylate, Gleevec). It shows 15 to 20 times greater efficacy in chronic myelogenous leukemia (CML) cell lines because it can bind to either open or closed activation loops. Bayard D. Clarkson, MD, presented the research at the Molecular Targets and Cancer Therapeutics meeting, sponsored by the American Association for Cancer Research, National Cancer Institute, and Europe- an Organization for Research and Treatment of Cancer (abstract 568). "We basically pulled this compound out of the literature, synthesized it, and found that it worked," said Dr. Clarkson, Enid A. Haupt Chair of Therapeutic Research and head of the Hematopoietic Cell Kinetics Laboratory at Memorial Sloan-Kettering Cancer Center. The MSKCC researchers collaborated with Dr. John Kuriyan and other Rockefeller University investigators on the x-ray crystal structure work. The initial translocation between chromosomes 9 and 22 in CML produces a BCR-encoded sequence fused to a truncated c-Abl gene. The Bcr- Abl protein encoded by this mutation greatly increases c-Abl's tyrosine kinase activity and causes the clinical manifestations of CML. STI-517 produces complete hematologic responses in most cases of CML because it binds to an ATP binding pocket and blocks this process. Dr. Clarkson's team set out to find inhibitors of Bcr-Abl that might be more effective than STI-571, which is met by resistance in patients whose CML is in blastic phase. PD17 is more effective than STI- 571 at inhibiting cell lines containing Bcr-Abl in vivo and at blocking CML progenitor cells without inhibiting normal progenitor cells. In vitro assays showed that PD17 inhibited fresh CML primary progenitor cells in the low nanomolar range. Dr. Clarkson said that PD17 is active and tolerable in mice at levels that should be active in humans. He presented x-ray crystal structures of the catalytic domain of c-Abl (Abl kinase) complexed with either STI-571 or PD17. These showed that STI-571 can bind to the target site only when the 21- residue activation loop of c-Abl is in a closed configuration that resembles substrate binding. PD17 binds when the activation loop is open and resembles that of an active kinase. "Modeling shows that PD17 can probably target c-Abl regardless of the activation state it is in, but STI- 571 can only recognize the unique downregulated form. The greater potency of PD17 is probably due, at least in part, to its ability to recognize multiple states of the kinase. We're not sure that is the entire reason, however," Dr. Clarkson said. Knowledge of the three-dimensional structure of the Abl kinase domain and how the specific inhibitors differ in their binding properties at a molecular level should allow scientists to make rational modifications to design and produce even more potent and specific drugs for treatment of leukemia and other types of cancer, he said. "I think that these tumors are going to develop resistance even to this drug [PD17], so we need more than one molecular target. I think that if we had three targets, as we do in promyelocytic leukemia, there is a good possibility that we could cure these tumors," he said. Dr. Clarkson also said that his group has studied another ParkeDavis (Pfizer) compound, PD166326, that is about fourfold more potent than PD17 in inhibiting Abl kinase. It is now being studied in CML mouse models. "PD17 and PD16 are also much more active than STI-571 against a variety of human glioblastoma, sarcoma, and neuroblastoma cell lines, but higher concentrations are needed than in the case of CML cells expressing Bcr-Abl," Dr. Clarkson said.