The platelet-derived growth factor receptor as a therapeutic target

Negative regulation of receptor tyrosine kinases by ubiquitination: Key roles of the Cbl family of E3 ubiquitin ligases

Receptor tyrosine kinases (RTKs) serve as transmembrane receptors that participate in a broad spectrum of cellular processes including cellular growth, motility, differentiation, proliferation, and metabolism. Hence, elucidating the regulatory mechanisms of RTKs involved in an assortment of diseases such as cancers attracts increasing interest from researchers. Members of the Cbl family ubiquitin ligases (c-Cbl, Cbl-b and Cbl-c in mammals) have emerged as negative regulators of activated RTKs. Upon activation of RTKs by growth factors, Cbl binds to RTKs via its tyrosine kinase binding (TKB) domain and targets them for ubiquitination, thus facilitating their degradation and negative regulation of RTK signaling. RTKs such as epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGF), fibroblast growth factor receptor (FGFR) and hepatocyte growth factor receptor (HGFR) undergo ubiquitination upon interaction with Cbl family members. In this review, we summarize the current knowledge related to the negative regulation of RTKs by Cbl family proteins.

Ligand-based computational modelling of platelet-derived growth factor beta receptor leading to new angiogenesis inhibitory leads

Platelet derived growth factor beta receptor (PDGFR- β) plays an important role in angiogenesis. PDGFR-β expression is correlated with increased vascularity and maturation of blood vessels in cancer. Pharmacophore modeling and quantitative structure-activity relationship (QSAR) analysis were combined to explore the structural requirements for ligand-PDGFR-β recognition using 107 known PDGFR-β inhibitors. Genetic function algorithm (GFA) coupled to k nearest neighbor (kNN) and multiple linear regression (MLR) analysis were employed to generate predictive QSAR models based on optimal combinations of pharmacophores and physicochemical descriptors. The successful pharmacophores were complemented with exclusion spheres to optimize their receiver operating characteristic curve (ROC) profiles. The QSAR models and their associated pharmacophore hypotheses were validated by identification and experimental evaluation of new angiogenesis inhibitory leads retrieved from the National Cancer Institute (NCI) structural database. Two hits illustrated low micromolar IC50 values in two distinct anti-angiogenesis bioassays.

Post-transcriptional up-regulation of PDGF-C by HuR in advanced and stressed breast cancer

Breast cancer is a heterogeneous disease characterized by multiple genetic alterations leading to the activation of growth factor signaling pathways that promote cell proliferation. Platelet-derived growth factor-C (PDGF-C) is overexpressed in various malignancies; however, the involvement of PDGF-C in breast cancers and the mechanisms underlying PDGF-C deregulation remain unclear. Here, we show that PDGF-C is overexpressed in clinical breast cancers and correlates with poor prognosis. PDGF-C up-regulation was mediated by the human embryonic lethal abnormal vision-like protein HuR, which stabilizes the PDGF-C transcript by binding to two predicted AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). HuR is up-regulated in hydrogen peroxide-treated or ultraviolet-irradiated breast cancer cells. Clinically, HuR levels are correlated with PDGF-C expression and histological grade or pathological tumor-node-metastasis (pTNM) stage. Our findings reveal a novel mechanism underlying HuR-mediated breast cancer progression, and suggest that HuR and PDGF-C are potential molecular candidates for targeted therapy of breast cancers.

Discovery of a Novel Class of Imidazo[1,2-a]Pyridines with Potent PDGFR Activity and Oral Bioavailability

The in silico construction of a PDGFRβ kinase homology model and ensuing medicinal chemistry guided by molecular modeling, led to the identification of potent, small molecule inhibitors of PDGFR. Subsequent exploration of structure-activity relationships (SAR) led to the incorporation of a constrained secondary amine to enhance selectivity. Further refinements led to the integration of a fluorine substituted piperidine, which resulted in significant reduction of P-glycoprotein (Pgp) mediated efflux and improved bioavailability. Compound 28 displayed oral exposure in rodents and had a pronounced effect in a pharmacokinetic-pharmacodynamic (PKPD) assay.

Virtual screening of selective multitarget kinase inhibitors by combinatorial support vector machines

Multitarget agents have been increasingly explored for enhancing efficacy and reducing countertarget activities and toxicities. Efficient virtual screening (VS) tools for searching selective multitarget agents are desired. Combinatorial support vector machines (C-SVM) were tested as VS tools for searching dual-inhibitors of 11 combinations of 9 anticancer kinase targets (EGFR, VEGFR, PDGFR, Src, FGFR, Lck, CDK1, CDK2, GSK3). C-SVM trained on 233-1,316 non-dual-inhibitors correctly identified 26.8%-57.3% (majority >36%) of the 56-230 intra-kinase-group dual-inhibitors (equivalent to the 50-70% yields of two independent individual target VS tools), and 12.2% of the 41 inter-kinase-group dual-inhibitors. C-SVM were fairly selective in misidentifying as dual-inhibitors 3.7%-48.1% (majority <20%) of the 233-1,316 non-dual-inhibitors of the same kinase pairs and 0.98%-4.77% of the 3,971-5,180 inhibitors of other kinases. C-SVM produced low false-hit rates in misidentifying as dual-inhibitors 1,746-4,817 (0.013%-0.036%) of the 13.56 M PubChem compounds, 12-175 (0.007%-0.104%) of the 168 K MDDR compounds, and 0-84 (0.0%-2.9%) of the 19,495-38,483 MDDR compounds similar to the known dual-inhibitors. C-SVM was compared to other VS methods Surflex-Dock, DOCK Blaster, kNN and PNN against the same sets of kinase inhibitors and the full set or subset of the 1.02 M Zinc clean-leads data set. C-SVM produced comparable dual-inhibitor yields, slightly better false-hit rates for kinase inhibitors, and significantly lower false-hit rates for the Zinc clean-leads data set. Combinatorial SVM showed promising potential for searching selective multitarget agents against intra-kinase-group kinases without explicit knowledge of multitarget agents.

Phase I study of the safety, tolerability, and pharmacokinetics of oral CP-868,596, a highly specific platelet-derived growth factor receptor tyrosine kinase inhibitor in patients with advanced cancers

PURPOSE

This phase I, first-in-human study evaluated the safety, tolerability, pharmacokinetics, and maximum-tolerated dose (MTD) of an oral platelet-derived growth factor receptor inhibitor, CP-868,596.

PATIENTS AND METHODS

Patients with advanced solid tumors were eligible. Dose escalations were performed in three groups with two formulations: uncoated on an empty stomach (UES), uncoated with food (UFED), and film-coated (FC) without food. Initial dose escalation in the UES group was followed by parallel escalations in the UFED and FC groups.

RESULTS

Fifty-nine patients enrolled. CP-868,596 was escalated from 100 mg to 340 mg daily in the UES group, from 60 mg to 100 mg twice daily in the UFED group, and from 100 mg once daily to 140 mg twice daily in the FC group. MTDs were 200 mg daily in the UES group and 100 mg twice daily in the FC group; MTD was not reached at 100 mg twice daily in the UFED group. Dose-limiting toxicities included hematuria, increased gamma-glutamyltransferase or ALT, insomnia, and nausea/vomiting. Most treatment-related AEs were of grades 1 to 2 severity; nausea, vomiting, and diarrhea were reported most frequently. Administration with food generally improved tolerability. CP-868,596 was absorbed slowly; systemic exposure parameters appeared to increase greater than proportionally with dose. Mean serum concentrations exceeded the preclinically predicted minimal efficacious concentration (ie, 16 ng/mL) at all dosages. Food and film coating apparently increased interpatient variability of the maximum observed plasma concentration and the area under the concentration-time curve. No objective responses were reported, and eight patients achieved stable disease (mean duration, 5.7 months).

CONCLUSION

CP-868,596 potentially demonstrated greater than dose-proportional pharmacokinetics. The recommended dosage of 100 mg twice daily with food was well tolerated. Additional development as a single agent in selected populations or in combination with chemotherapy in broader populations is warranted.

The expression and prognostic significance of platelet-derived growth factor receptor alpha in mature T- and natural killer-cell lymphomas

Platelet-derived growth factor receptor alpha (PDGFRA) is important in numerous malignancies and can serve as a target for therapeutic strategy. We aimed to assess the role of PDGFRA expression in mature T- and natural killer (NK)-cell lymphomas. We used immunohistochemistry to analyze PDGFRA expression in mature T- and NK-cell lymphomas in tissue samples from 50 patients. In positive samples, we then did a mutational analysis of the PDGFRA gene on exons 10, 12, 14, and 18. The relationship between PDGFRA expression and overall survival in mature T- and NK-cell lymphomas was statistically analyzed in the study. We analyzed PDGFRA expression in four subtypes: angioimmunoblastic T-cell lymphoma (3/8, 37.5%); anaplastic large cell lymphoma (5/7, 71.4%); NK/T-cell lymphoma, nasal type (9/12, 75%); and peripheral T-cell lymphoma, unspecified (PTCLu; 7/23, 30.4%). It was lower in PTCLu than in other subtypes (30.4% vs. 63%, p=0.022). PDGFRA expression was not related to PDGFRA gene mutation. Overall survival in mature T- and NK-cell lymphomas correlated significantly with disease subtypes and an international prognostic index but not PDGFRA expression. Our study showed that PDGFRA expression was different in mature T- and NK-cell lymphomas. PDGFRA expression in PTCLu was lower in the present study than in previous reports done in Western countries. It suggests that this disease is biologically distinct in different races (30.4% vs. 91-100%).