A systematic analysis reveals that out of 20 protein kinases examined, specific for either Ser/Thr or Tyr, the majority are extremely sensitive to staurosporine, with IC,,, values in the low nanomolar range. A few of them however, notably protein kinases CKI and CK2, mitogen-activated protein (MAP) kinase and protein-tyrosine kinase CSK, are relatively refractory to staurosporine inhibition, exhibiting IC,,, values in the micromolar range. With all protein kinases tested. namely PKA, CKI, CK2, MAP kinase (ERK-I), c-Fgr, Lyn, CSK and TPK-IIB/p38.'", staurosporine inhibition was competitive with respect to ATP, regardless of its inhibitory power. In contrast, either uncompetitive or noncompetitive kinetics of inhibition with respect to the phosphoacceptor substrate were exhibited by Ser/Thr and Tyr-specific protein kinases, respectively, consistent with a different mechanism of catalysis by these two sub-families of kinases. Computer modeling based on PKA crystal structure in conjunction with sequence analysis suggest that the low sensitivity to staurosporine of CK2 may be accounted for by the bulky nature of three residues, Va166, P h e l l 3 and Ile174 which are homologous to PKA Ala70, Met120 and Thr183, respectively. In contrast these PKA residues are either conserved or replaced by smaller ones in protein kinases highly sensitive to staurosporine inhibition. On the other hand, His160 which is homologous to PKA Glu170, appears to be responsible for the unique behaviour of CK2 with respect to a staurosporine derivative (CGP44171A) bearing a negatively charged benzoyl substituent: while CGP44171A is 10-100-fold less effective than staurosporine against PKA and most of the other protein kinases tested, it is actually more effective than staurosporine for CK2 inhibition, but it looses part of its efficacy if it is tested on a CK2 mutant (H160D) in which His160 has been replaced by Asp.It can be concluded from these data that the catalytic sites of protein kinases are divergent enough as to allow a competitive inhibitor like staurosporine to be fairly selective, a feature that can be enhanced by suitable modifications designed based on the structure of the catalytic site of the kinase.

In the course of the random screening of a pool of CIBA chemicals, the two pyrazolopyrimidines 1 and 2 have been identified as fairly potent inhibitors of the EGF-R tyrosine kinase. Using a pharmacophore model for ATP-competitive inhibitors interacting with the active site of the EGF-R protein tyrosine kinase (PTK), the class of the pyrazolo[3,4-d]pyrimidines was then optimized in an interactive process leading to a series of 4-(phenylamino)-1H-pyrazolo[3,4-d]-pyrimidines as highly potent inhibitors of the EGF-R tyrosine kinase. The most potent compounds 13, 14, 15, 17, 19, 22, 26, 28, and 30 of this series inhibited the EGF-R PTK with IC50 values below 10 nM. High selectivity toward a panel of nonreceptor tyrosine kinases (c-Src, v-Abl and serine/threonine kinases (PKC alpha, CDK1) was observed. In cells, EGF-stimulated cellular tyrosine phosphorylation was inhibited by compounds 13, 15, 19, 22, and 23 at IC50 values below 50 nM, whereas PDGF-induced tyrosine phosphorylation was not affected by concentrations up to 10 microM, thus indicating high selectivity for the inhibition of the ligand-activated EGF-R signal transduction pathway. Compounds 15 and 19 inhibited proliferation of the EGF-dependent MK cell line with IC50 values below 0.5 microM. In addition, two compounds, 9 and 11, showing satisfactory oral bioavailability in mice after oral administration, exhibited good in vivo efficacy at doses of 12.5 and 50 mg/kg in a nude mouse tumor model using xenografts of the EGF-R overexpressing A431 cell line. From SAR studies, a binding mode for 4-(phenylamino)-1H-pyrazolo[3,4-d]pyrimidines, especially for compound 15, at the ATP-binding site of the EGF-R tyrosine kinase is proposed. 4-(Phenylamino)-1H-pyrazolo[3,4-d]pyrimidines represent a new class of highly potent tyrosine kinase inhibitors which preferentially inhibit the EGF-mediated signal transduction pathway and have the potential for further evaluation as anticancer agents.

Using a pharmacophore model for ATP-competitive inhibitors interacting with the active site of the EGF-R protein tyrosine kinase (PTK), 4-(phenylamino)-7H-pyrrolo[2,3-d]pyrimidines have been identified as a novel class of potent EGF-R protein tyrosine kinase inhibitors. In an interactive process, this class of compounds was then optimized. 13, 14, 28, 36, 37, and 44, the most potent compounds of this series, inhibited the EGF-R PTK with IC50 values in the low nanomolar range. High selectivity toward a panel of nonreceptor tyrosine kinases (c-Src, v-Abl) and serine/threonine kinases (PKC alpha, PKA) was observed. Kinetic analysis revealed competitive type kinetics relative to ATP. In cells, EGF-stimulated cellular tyrosine phosphorylation was inhibited by compounds 13, 36, 37, and 44 at IC50 values between 0.1 and 0.4 microM, whereas PDGF-induced tyrosine phosphorylation was not affected by concentrations up to 10 microM. In addition, these compounds were able to selectively inhibit c-fos mRNA expression in EGF-dependent cell lines with IC50 values between 0.1 and 2 microM, but did not affect c-fos mRNA induction in response to PDGF or PMA (IC50 >100 microM). Proliferation of the EGF-dependent MK cell line was inhibited with similar IC50 values. From SAR studies, a binding mode for 4-(phenylamino)-7H-pyrrolo[2,3-d]pyrimidines as well as for the structurally related 4-(phenylamino)quinazolines at the ATP-binding site of the EGF-R tyrosine kinase is proposed. 4-(Phenylamino)7H-pyrrolo[2,3-d]pyrimidines therefore represent a new class of highly potent tyrosine kinase inhibitors which preferentially inhibit the EGF-mediated signal transduction pathway and have the potential for further evaluation as anticancer agents.

The platelet-derived growth factor (PDGF) receptor is a member of the transmembrane growth factor receptor protein family with intrinsic protein-tyrosine kinase activity. We describe a potent protein-tyrosine kinase inhibitor (CGP 53716) that shows selectivity for the PDGF receptor in vitro and in the cell. The compound shows selectivity for inhibition of PDGF-mediated events such as PDGF receptor autophosphorylation, cellular tyrosine phosphorylation, and c-fos mRNA induction in response to PDGF stimulation of intact cells. In contrast, ligand-induced autophosphorylation of the epidermal growth factor (EGF) receptor, insulin receptor, and the insulin-like growth factor I receptor, as well as c-fos mRNA expression induced by EGF, fibroblast growth factor, and phorbol ester, was insensitive to inhibition by CGP 53716. In antiproliferative assays, the compound was approximately 30-fold more potent in inhibiting PDGF-mediated growth of v-sis-transformed BALB/c 3T3 cells relative to inhibition of EGF-dependent BALB/Mk cells, interleukin-3-dependent FDC-P1 cells, and the T24 bladder carcinoma line. When tested in vivo using highly tumorigenic v-sis- and human c-sis-transformed BALB/c 3T3 cells, CGP 53716 showed antitumor activity at well-tolerated doses. In contrast, CGP 53716 did not show antitumor activity against xenografts of the A431 tumor, which overexpresses the EGF receptor. These findings suggest that CGP 53716 may have therapeutic potential for the treatment of diseases involving abnormal cellular proliferation induced by PDGF receptor activation.

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