Soluble 2-substituted aminopyrido[2,3-d]pyrimidin-7-yl ureas. Structure-activity relationships against selected tyrosine kinases and exploration of in vitro and in vivo anticancer activity

In continuing our search for medicinal agents to treat proliferative diseases, we have discovered 2-substituted aminopyrido[2,3-d]pyrimidin-7-yl ureas as a novel class of soluble, potent, broadly active tyrosine kinase (TK) inhibitors. An efficient route was developed that enabled the synthesis of a wide variety of analogues with substitution on several positions of the template. From the lead structure 1, several series of analogues were made that examined the C-6 aryl substituent, a variety of water solublizing substitutents at the C-2 position, and urea or other acyl functionality at the N-7 position. Compounds of this series were competitive with ATP and displayed submicromolar to low nanomolar potency against a panel of TKs, including receptor (platelet-derived growth factor, PDGFr; fibroblast growth factor, FGFr;) and nonreceptor (c-Src) classes. Several of the most potent compounds displayed submicromolar inhibition of PDGF-mediated receptor autophosphorylation in rat aortic vascular smooth muscle cells and low micromolar inhibition of cellular growth in five human tumor cell lines. One of the more thoroughly evaluated members, 32, with IC50 values of 0.21 microM (PDGFr), 0.049 microM (bFGFr), and 0.018 microM (c-Src), was evaluated in in vivo studies against a panel of five human tumor xenografts, with known and/or inferred dependence on the EGFr, PDGFr, and c-Src TKs. Compound 32 produced a tumor growth delay of 14 days against the Colo-205 colon xenograft model.

Structure-activity relationships for 1-phenylbenzimidazoles as selective ATP site inhibitors of the platelet-derived growth factor receptor

1-Phenylbenzimidazoles are shown to be a new class of ATP-site inhibitors of the platelet-derived growth factor receptor (PDGFR). Structure-activity relationships (SARs) are narrow, with closely related heterocycles being inactive. A systematic study of substituted 1-phenylbenzimidazoles showed clear SARs. Substituents at the 4'- and 3'-positions of the phenyl ring are tolerated but do not significantly improve activity, while substituents at the 2'-position abolish it. Substituents in the 2-, 4-, and 7-positions of the benzimidazole ring (with the exception of 4-OH) also abolish activity. Most substituents at the 5- and 6-positions maintain or increase activity, with the 5-OH, 5-OMe, 5-COMe, and 5-CO2Me analogues being >10-fold more potent than the parent 1-phenylbenzimidazole. The 5-OMe analogue was both the most potent inhibitor, and showed the highest selectivity (50-fold) between PDGFR and FGFR isolated enzymes, and also a moderately effective inhibitor (IC50 = 1.9 microM) of PDGF-stimulated PDGFR autophosphorylation in rat aorta smooth muscle cells.

Design of peptidomimetics that inhibit the association of phosphatidylinositol 3-kinase with platelet-derived growth factor-beta receptor and possess cellular activity

Phosphorylated tyrosine residues of growth factor receptors that associate with intracellular proteins containing src-homology 2 (SH2) domains are integral components in several signal transduction pathways related to proliferative diseases such as cancer, atherosclerosis, and restenosis. In particular, a phosphorylated pentapeptide [pTyr751-Val-Pro-Met754-Leu (pTyr = phosphotyrosine)] derived from the primary sequence of platelet-derived growth factor-beta (PDGF-beta) receptor blocks the association of the C-terminal SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) to PDGF-beta receptor with an IC50 of 0.445 +/- 0.047 microM. Further evaluation of the structure-activity relationships for pTyr751-Val-Pro-Met-Leu resulted in the design of smaller peptidomimetics with enhanced affinity including Ac-pTyr-Val-Ala-N(C6H13)2 (IC50 = 0.076 +/- 0.010 microM). In addition, the phosphotyrosine residue was replaced with a difluorophosphonate derivative [4-phosphono(difluoromethyl)phenylalanine (CF2Pmp)] which has been shown to be stable to cellular phosphatases. The extracellular administration of either CF2Pmp-Val-Pro-Met-Leu or Ac-CF2Pmp-Val-Pro-Met-NH2 in a whole cell assay resulted in specific inhibition of the PDGF-stimulated association from the C-terminal SH2 domain of the p85 subunit of PI 3-kinase to the PDGF-beta receptor in a dose-dependent manner. These compounds were also effective in inhibiting GLUT4 translocation, c-fos expression, and cell membrane ruffling in single-cell microinjection assay.

Synthesis and tyrosine kinase inhibitory activity of a series of 2-amino-8H-pyrido[2,3-d]pyrimidines: identification of potent, selective platelet-derived growth factor receptor tyrosine kinase inhibitors

Screening of a compound library led to the identification of 2-amino-6-(2,6-dichlorophenyl)-8-methylpyrido[2,3-d]pyrimidine (1) as a inhibitor of the platelet-derived growth factor receptor (PDGFr), fibroblast growth factor receptor (FGFr), and c-src tyrosine kinases (TKs). Replacement of the primary amino group at C-2 of 1 with a 4-(N,N-diethylaminoethoxy)phenylamino group yielded 2a, which had greatly increased activity against all three TKs. In the present work, variation of the aromatic group at C-6 and of the alkyl group at N-8 of the pyrido[2,3-d]pyrimidine core provided several analogues that retained potency, including derivatives that were biased toward inhibition of the TK activity of PDGFr. Analogues of 2a with a 3-thiophene or an unsubstituted phenyl group at C-6 were the most potent inhibitors. Compound 54, which had IC50 values of 31, 88, and 31 nM against PDGFr, FGFr, and c-src TK activity, respectively, was active in a variety of PDGF-dependent cellular assays and blocked the in vivo growth of three PDGF-dependent tumor lines.

2-Substituted aminopyrido[2,3-d]pyrimidin-7(8H)-ones. structure-activity relationships against selected tyrosine kinases and in vitro and in vivo anticancer activity

While engaged in therapeutic intervention against a number of proliferative diseases, we have discovered the 2-aminopyrido[2, 3-d]pyrimidin-7(8H)-ones as a novel class of potent, broadly active tyrosine kinase (TK) inhibitors. An efficient route was developed that enabled the synthesis of a wide variety of analogues with substitution on several positions of the template. From the lead structure 2, a series of analogues bearing variable substituents at the C-2 position and methyl or ethyl at N-8 was made. Compounds of this series were competitive with ATP and displayed submicromolar to low nanomolar potency against a panel of TKs, including receptor (platelet-derived growth factor, PDGFr; fibroblast growth factor, FGFr; epidermal growth factor, EGFr) and nonreceptor (c-Src) classes. One of the more thoroughly evaluated members was 63 with IC50 values of 0.079 microM (PDGFr), 0.043 microM (bFGFr), 0.044 microM (EGFr), and 0.009 microM (c-Src). In cellular studies, 63 inhibited PDGF-mediated receptor autophosphorylation in a number of cell lines at IC50 values of 0.026-0.002 microM and proliferation of two PDGF-dependent lines at 0.3 microM. It also caused inhibition of soft agar colony formation in three cell lines that overexpress the c-Src TK, with IC50 values of 0.33-1.8 microM. In in vivo studies against a panel of seven xenograft tumor models with known and/or inferred dependence on the EGFr, PDGFr, and c-Src TKs, compound 63 produced a tumor growth delay of 10.6 days against the relatively refractory SK-OV-3 ovarian xenograft and also displayed activity against the HT-29 tumor. In rat oral bioavailability studies, compound 63 plasma concentrations declined in a biexponential manner, and systemic plasma clearance was high relative to liver blood flow. Finally, in rat metabolism studies, HPLC chromatography identified two metabolites of 63, which were proved by mass spectrometry and synthesis to be the primary amine (58) and N-oxide (66). Because of the excellent potency of 63 against selected TKs, in vitro and in vivo studies are underway for this compound in additional tumor models dependent upon PDGFr, FGFr, and c-Src to assess its potential for advancement to clinical trials.

In vitro biological characterization and antiangiogenic effects of PD 166866, a selective inhibitor of the FGF-1 receptor tyrosine kinase

Through direct synthetic efforts, we discovered a small molecule that is a nanomolar inhibitor of the human fibroblast growth factor-1 receptor (FGFR) tyrosine kinase. PD 166866, a member of a new structural class of tyrosine kinase inhibitors, the 6-aryl-pyrido[2,3-d]pyrimidines, was identified by screening a compound library with assays that measure protein tyrosine kinase activity. PD 166866 inhibited human full-length FGFR-1 tyrosine kinase with an IC50 value of 52.4 +/- 0.1 nM and was further characterized as an ATP competitive inhibitor of the FGFR-1. In contrast, PD 166866 had no effect on c-Src, platelet-derived growth factor receptor-beta, epidermal growth factor receptor or insulin receptor tyrosine kinases or on mitogen-activated protein kinase, protein kinase C and CDK4 at concentrations as high as 50 microM. PD 166866 was a potent inhibitor of basic fibroblast growth factor (bFGF)-mediated receptor autophosphorylation in NIH 3T3 cells expressing endogenous FGFR-1 and in L6 cells overexpressing the human FGFR-1 tyrosine kinase, confirming a tyrosine kinase-mediated mechanism. PD 166866 also inhibited bFGF-induced tyrosine phosphorylation of the 44- and 42-kDa (ERK 1/2) mitogen-activated protein kinase isoforms in L6 cells, presumably via inhibition of bFGF-stimulated FGFR-1 tyrosine kinase activation. PD 166866 did not inhibit platelet-derived growth factor, epidermal growth factor or insulin-stimulated receptor autophosphorylation in vascular smooth muscle, A431 or NIHIR cells, respectively, further supporting its specificity for the FGFR-1. In addition, daily exposure of PD 166866 to L6 cells at concentrations from 1 to 100 nM resulted in a concentration-related inhibition of bFGF-stimulated cell growth for 8 consecutive days with an IC50 value of 24 nM. In contrast, PD 166866 had little effect on platelet-derived growth factor-BB-stimulated growth of L6 cells or serum-stimulated vascular smooth muscle cell proliferation. Finally, PD 166866 was found to be a potent inhibitor of microvessel outgrowth (angiogenesis) from cultured artery fragments of human placenta. These results highlight the discovery of PD 166866, a new nanomolar potent and selective small molecule inhibitor of the FGFR-1 tyrosine kinase with potential use as antiproliferative/antiangiogenic agent for such therapeutic targets as tumor growth and neovascularization of atherosclerotic plaques.

Inhibition of FGF-1 receptor tyrosine kinase activity by PD 161570, a new protein-tyrosine kinase inhibitor

Through direct synthetic efforts we discovered a small molecule which is a 40 nanomolar inhibitor of the human FGF-1 receptor tyrosine kinase. 1-Tert-butyl-3-[6-(2,6-dichloro-phenyl)-2-(4-diethylamino-butylamino)-py rido[2,3-d]pyrimidin-7-yl]-urea (PD 161570) had about 5- and 100-fold greater selectivity toward the FGF-1 receptor (IC50 = 40 nM) compared with the PDGFbeta receptor (IC50 = 262 nM) or EGF receptor (IC50 = 3.7 microM) tyrosine kinases, respectively. In addition, PD 161570 suppressed constitutive phosphorylation of the FGF-1 receptor in both human ovarian carcinoma cells (A121(p)) and Sf9 insect cells overexpressing the human FGF-1 receptor and blocked the growth of A121(p) cells in culture. The results demonstrate a novel synthetic inhibitor with nanomolar potency and specificity towards the FGF-1 receptor tyrosine kinase.

In vitro pharmacological characterization of PD 166285, a new nanomolar potent and broadly active protein tyrosine kinase inhibitor

PD 166285, a novel protein tyrosine kinase inhibitor of a new structural class, the 6-aryl-pyrido[2,3-d]pyrimidines, was synthesized as the most potent and soluble analog of a series of small molecules originally identified by screening a compound library with assays that measured protein tyrosine kinase activity. PD 166285 was found to inhibit Src nonreceptor tyrosine kinase, fibroblast growth factor receptor-1, epidermal growth factor receptor and platelet-derived growth factor receptor beta subunit (PDGFR-beta), tyrosine kinases with half-maximal inhibitory potencies (IC50 values) of 8.4 +/- 2.3 nM (n = 6), 39.3 +/- 2.8 nM (n = 16), 87.5 +/- 13.7 nM (n = 6) and 98.3 +/- 7.9 nM (n = 16), respectively. PD 166285 also demonstrated inhibitory activity against mitogen-activated protein kinase (IC50 = 5 microM) and protein kinase C (IC50 = 22.7 microM). PD 166285 was further characterized as an ATP competitive inhibitor of Src nonreceptor tyrosine kinase, PDGFR-beta, fibroblast growth factor receptor-1 and epidermal growth factor receptor tyrosine kinases. In addition, PD 166285 inhibited PDGF- and EGF-stimulated receptor autophosphorylation in vascular smooth muscle cells (VSMCs) and A431 cells, respectively, and basic fibroblast growth factor-mediated tyrosine phosphorylation in Sf9 cells, with IC50 values of 6.5 nM, 1.6 microM and 97.3 nM, respectively, further establishing a tyrosine kinase mechanism of inhibition. The inhibition of PDGF receptor autophosphorylation in VSMCs by PD 166285 was long lasting and persisted for 4 days after a single 1-hr exposure followed by extensive washing. The PDGF-induced tyrosine phosphorylation of the 44- and 42-kDa mitogen-activated protein kinase isoforms was also blocked as a result of the inhibition of PDGF-stimulated receptor autophosphorylation by PD 166285 in VSMCs. The effects of PD 166285 were also demonstrated in functional assays of cell attachment, migration and proliferation, in which vascular cell adhesion to vitronectin, PDGF-directed chemotaxis and serum-stimulated cell growth were all potently inhibited with IC50 values of 80 yo 120 nM. Finally, PD 166285 uniquely demonstrated potent inhibition of phorbol ester-induced production of 92-kDa gelatinase A (MMP-9) in VSMC without affecting 72-kDa gelatinase B (MMP-2) as measured by gelatin zymography. These results highlight the biological characteristics of PD 166285 as a broadly active protein tyrosine kinase capable of potently inhibiting a number of kinase mediated cellular functions, including cell attachment, movement and replication. The potential therapeutic utility of this broadly acting inhibitor as an antiproliferative and antimigratory agent could extend to such diseases as cancer, atherosclerosis and restenosis, in which redundancies in protein kinase signaling pathways are known to exist.

PDGF receptor protein tyrosine kinase expression in the balloon-injured rat carotid artery

Platelet-derived growth factor (PDGF) receptor gene expression has previously been demonstrated in balloon-injured rat carotid arteries to be regulated during repair of carotid injury. In the present study we showed that PDGF receptor protein expression and phosphorylation are changed over time after carotid artery injury. In control and 2-day-postinjury vessels, expression of PDGF alpha receptor protein was readily detectable, whereas PDGF beta receptor expression appeared very low. Between 2 and 7 days postinjury, a time interval previously shown to correspond with smooth muscle cell migration followed by the appearance of a neointima, PDGF alpha receptor expression had increased only slightly, to roughly 35% above control levels, and was maximal by day 7 postinjury, whereas PDGF beta receptor expression had doubled. From 7 to 14 days after carotid injury, intimal area was greatly increased and was associated with a further increase in PDGF beta receptor protein expression and receptor phosphorylation to a maximum between days 10 and 12. In contrast, PDGF alpha receptor expression had decreased slightly during this time interval. Moreover, phosphorylation of PDGF alpha receptors was barely detectable and did not change over the time course of injury. From 14 to 28 days after injury, intimal area was increased only slightly, whereas PDGF beta receptor protein and phosphorylation levels had diminished to roughly half of the 10-day injury values. In addition, the increase in PDGF beta receptor protein expression and tyrosine phosphorylation observed over the time of injury were also associated with a corresponding increase in the association of phosphatidylinositol 3' kinase (PI-3 kinase) with phosphorylated PDGF beta receptors. These findings show that balloon injury to rat carotid arteries results in temporally related changes in the expression of PDGF receptors and their state of tyrosine phosphorylation. Furthermore, tyrosine phosphorylation of PDGF beta receptors in the balloon-injured rat carotid artery in vivo resulted in the association of PI-3 kinase. These are important new findings, which add to our knowledge concerning the role and activity of PDGF receptors in the formation of a neointima.

Inhibition of growth factor-mediated tyrosine phosphorylation in vascular smooth muscle by PD 089828, a new synthetic protein tyrosine kinase inhibitor

PD 089828, a novel protein tyrosine kinase inhibitor of a new structural class, the 6-aryl-pyrido-[2,3-d]pyrimidines, was identified by screening a compound library with assays that measured protein tyrosine kinase activity. PD 089828 was found to inhibit human full-length fibroblast growth factor (FGF) receptor-1 (FGFR-1), platelet-derived growth factor (PDGF) receptor beta subunit (PDGFR-beta), Src nonreceptor tyrosine kinase (c-Src) and epidermal growth factor (EGF) receptor (EGFR) tyrosine kinases with half-maximal inhibitory potencies (IC50 values) of 0.15 +/- 0.02 (n = 4), 0.18 +/- 0.04 (n = 3), 1.76 +/- 0.28 (n = 4) and 5.47 +/- 0.78 (n = 6) microM, respectively. PD 089828 was further characterized as an ATP competitive inhibitor of the growth factor receptor tyrosine kinases (FGFR-1, PDGFR-beta and EGFR) but a noncompetitive inhibitor of c-Src tyrosine kinase with respect to ATP. In addition, PD 089828 inhibited PDGF- and EGF-stimulated receptor autophosphorylation in vascular SMC (VSMC) and basic FGF-mediated tyrosine phosphorylation in A121 cells with IC50 values similar to the potencies observed for inhibition of receptor tyrosine kinase activity. The inhibition of PDGF receptor autophosphorylation in VSMC by PD 089828 occurred rapidly, with maximal effects reached within 5 min of drug exposure. Inhibition after single exposure was long lasting but also rapidly reversible, occurring within 5 min after drug removal. The PDGF-induced association of downstream signaling proteins, including phosphoinositide-3-kinase (PI-3K), growth factor receptor binding protein-2 (GRB2), SH-2 domain and collagen like (Shc) and phospholipase Cgamma (PLCgamma), with VSMC PDGF receptors was also blocked as a result of the inhibition of PDGF-stimulated receptor autophosphorylation by PD 089828. PD 089828 also inhibited the PDGF-induced tyrosine phosphorylation of the 44- and 42-kDa mitogen-activated protein kinase isoforms. Moreover, the effects of PD 089828 were demonstrated in functional assays in which PDGF-stimulated DNA synthesis, PDGF-directed migration and serum-stimulated growth of VSMC were all inhibited to the same extent as PDGF receptor autophosphorylation (IC50 = 0.8, 4.5 and 1.8 microM, respectively). These results highlight the biological characteristics of PD 089828 as a novel, broadly active protein tyrosine kinase inhibitor with long-lasting but reversible cellular effects. The potential therapeutic use of these broadly acting, nonselective inhibitors as antiproliferative and antimigratory agents could extend to such diseases as cancer, atherosclerosis and restenosis in which redundancies in growth-signaling pathways are known to exist.