A rational design-based approach to developing targeted therapeutics has generated a small-molecule inhibitor that blocks both PI(3)K and tyrosine kinase activity
Tyrosine kinases stimulate the Raf-MEK-Erk and phosphatidylinositol-3-OH kinase (PI(3)K) pathways and are mutationally activated in many human cancers. Small-molecule tyrosine-kinase inhibitors such as imatinib — which targets the oncogenic Bcr-Abl fusion protein and select receptor tyrosine kinases — have been used with clinical success. However, some cancers possess or acquire resistance to tyrosine kinase inhibition through drug-resistant mutations or activating mutations in PI(3)K pathway members. Thus, there is considerable interest in developing multi-kinase inhibitors that target the PI(3)K-mTOR (mammalian target of rapamycin) and tyrosine kinase pathways. In Nature Chemical Biology, Apsel et al. now report the generation of a novel pyrazolopyrimidine compound that inhibits tyrosine and PI(3) kinases and blocks the proliferation of many tumor cell lines, including imatinib-resistant chronic myelogenous leukemia (CML) cells.
Tyrosine kinases and PI(3)Ks lack primary sequence homology, but share a similar kinase domain architecture, suggesting that a single small molecule could target both families. To generate novel compounds with dual specificity, the authors screened a tyrosine kinase inhibitor library for compounds that blocked PI(3)K activity, and then performed structure-activity relationship (SAR) studies to increase potency and selectivity. One of the resulting novel compounds, PP121, exhibited nanomolar efficacy against both PI(3)K and tyrosine kinases. Importantly, PP121 did not inhibit serine/threonine kinase activity, an effect that was ascribed to a bulky 'gatekeeper' amino acid in the kinases that blocked inhibitor access to the ATP-binding pocket.
PP121 treatment inhibited the phosphorylation of PI(3)K-mTOR pathway members and induced G0/G1 cell cycle arrest of cancer cell lines containing mutations in PTEN (phosphatase and tensin homolog), the p110α PI(3)K catalytic subunit and Ras. However, PP121 also exhibited potent activity against tyrosine kinases. It blocked tyrosine phosphorylation and reversed morphological transformation induced by v-Src, as well as inhibiting Ret and VEGFR2 autophosphorylation and pathway activation.
Given that PP121 can inhibit both tyrosine kinases and PI(3)Ks, the authors next evaluated whether PP121 could block the growth of imatinib-resistant CML. Drug resistance in CML is achieved through a T315I mutation in Bcr-Abl (Bcr-AblT315I). Remarkably, even though PP121 did not inhibit Bcr-AblT315I-mediated tyrosine phosphorylation, it nevertheless blocked cellular proliferation by inducing G0/G1 cell cycle arrest. As the mTOR substrate S6 was not phosphorylated in these cells, the authors attributed this effect to PI(3)K-mTOR pathway inhibition.
This study provides compelling evidence that a rational design-based approach to targeting two signaling pathways can generate promising new anti-cancer drugs. It will be interesting to determine whether this technique could be applied to selectively inhibit other combinations of kinases.
Emily J. Chenette Signaling Gateway
References
Apsel, B. et al. Targeted polypharmacology: discovery of dual inhibitors of tyrosine and phosphoinositide kinases Nature Chemical Biology4, 691-699 (2008) | Article | PubMed |
Bilanges, B., Torbett, N. & Vanhaesebroeck, B. Killing two kinase families with one stone. Nature Chemical Biology4, 648-649 (2008) | Article | PubMed |
