CT53518, a novel selective FLT3 antagonist for the treatment of acute myelogenous leukemia (AML).

Up to 30% of acute myelogenous leukemia (AML) patients harbor an activating internal tandem duplication (ITD) within the juxtamembrane domain of the FLT3 receptor, suggesting that it may be a target for kinase inhibitor therapy. For this purpose we have developed CT53518, a potent antagonist that inhibits FLT3, platelet-derived growth factor receptor (PDGFR), and c-Kit (IC(50) approximately 200 nM), while other tyrosine or serine/threonine kinases were not significantly inhibited. In Ba/F3 cells expressing different FLT3-ITD mutants, CT53518 inhibited IL-3-independent cell growth and FLT3-ITD autophosphorylation with an IC(50) of 10-100 nM. In human FLT3-ITD-positive AML cell lines, CT53518 induced apoptosis and inhibited FLT3-ITD phosphorylation, cellular proliferation, and signaling through the MAP kinase and PI3 kinase pathways. Therapeutic efficacy of CT53518 was demonstrated both in a nude mouse model and in a murine bone marrow transplant model of FLT3-ITD-induced disease.

Platelet-derived growth factor (PDGF) autocrine signaling regulates survival and mitogenic pathways in glioblastoma cells: evidence that the novel PDGF-C and PDGF-D ligands may play a role in the development of brain tumors.

Glioblastoma multiforme, the most common form of malignant brain tumor,is resistant to all forms of therapy and causes death within 9-12 months of diagnosis. Glioblastomas are known to contain numerous genetic and physiological alterations affecting cell survival and proliferation; one of the most common alterations being platelet-derived growth factor (PDGF) autocrine signaling characterized by coexpression of PDGF and its receptor. The PDGF family consists of four members, PDGF-A, -B, -C, and -D, that signal through the alpha and beta PDGF receptor (PDGFR) tyrosine kinases. Numerous studies have demonstrated expression of PDGF-A, PDGF-B, and the PDGFRs in gliomablastomas, but such studies have not been conducted for the newly identified PDGF-C and -D. Therefore, we examined the expression of all PDGF ligands and receptors in 11 glioma cell lines and 5 primary glioblastoma tumor tissues by quantitative reverse transcription-PCR. Expression of PDGF/PDGFR pairs that are known to functionally interact were identified in all of the samples. Interestingly, PDGF-C expression was ubiquitous in brain tumor cells and tissues but was very low or absent in normal adult and fetal brain. PDGF-D was expressed in 10 of 11 brain tumor cell lines and 3 of 5 primary brain tumor samples. As a strategy for blocking PDGFR signaling, CT52923, a potent selective small molecule piperazinyl quinazoline kinase inhibitor of the PDGFR, was identified. In model systems using NIH/3T3 cells, CT52923 blocked PDGF autocrine-mediated phosphorylation of PDGFR, Akt, and mitogen-activated protein kinase (MAPK), while having no effect on v-fms or V12-ras-mediated Akt or extracellular signal-regulated protein kinase (Erk) phosphorylation. More importantly, p.o. administration of CT52923 to nude mice caused a significant 61% reduction (P < 0.006) in tumor growth of NIH/3T3 cells transformed by PDGF, whereas tumor formation by cells expressing v-fms was unaffected. We next characterized PDGF autocrine signaling in five glioblastoma cell lines. In all of the cases, PDGF autocrine signaling was evident because treatment with 1-10 microM CT52923 inhibited PDGFR autophosphorylation when present at a detectable level and blocked downstream Akt and/or Erk phosphorylation. The functional significance of PDGF autocrine signaling in these cells was demonstrated by the fact that the CT52923 inhibited soft agar colony formation, and, when given p.o. to nude mice, it effectively reduced tumor formation by 44% (P < 0.0019) after s.c. injection of C6 glioblastoma cells. This study of glioblastoma cells and primary tissues is the first to implicate PDGF-C and -D in brain tumor formation and confirms the existence of autocrine signaling by PDGF-A and -B. More importantly, treatment with the PDGFR antagonist CT52923 inhibited survival and/or mitogenic pathways in all of the glioblastoma cell lines tested and prevented glioma formation in a nude mouse xenograft model. Together these findings demonstrate the potential therapeutic utility of this class of compounds for the treatment of glioblastoma.

PDGF C is a selective alpha platelet-derived growth factor receptor agonist that is highly expressed in platelet alpha granules and vascular smooth muscle.

OBJECTIVE: The platelet-derived growth factor (PDGF) family consists of four members, PDGF A, PDGF B, and 2 new members, PDGF C and PDGF D, which signal through the alpha and beta PDGF receptor (PDGFR) tyrosine kinases. This study was performed to determine the receptor specificity and cellular expression profile of PDGF C. METHODS AND RESULTS: PDGF C growth factor domain (GFD) was shown to preferentially bind and activate alpha PDGFR and activate beta PDGFR when it is co-expressed with alpha PDGFR through heterodimer formation. An investigation of PDGF C mRNA and protein expression revealed that during mouse fetal development, PDGF C was expressed in the mesonephric mesenchyme, prefusion skeletal muscle, cardiac myoblasts, and in visceral and vascular smooth muscle, whereas in adult human tissues expression was largely restricted to smooth muscle. Microarray analysis of various cell types showed PDGF C expression in vascular smooth muscle cells, renal mesangial cells, and platelets. PDGF C mRNA expression in platelets was confirmed by real-time polymerase chain reaction, and PDGF C protein was localized in alpha granules by immuno-gold electron microscopy. Western blot analysis of platelets identified 55-kDa and 80-kDa PDGF C isoforms that were secreted on platelet activation. CONCLUSIONS: Taken together, our results demonstrated for the first time to our knowledge that like PDGF A and B, PDGF C is likely to play a role in platelet biology.