Targeted therapies directed against receptor-activated protein kinases have transformed clinical management of many cancers, but keeping a step ahead of inevitable cancer resistance to these drugs resembles the game of whack-a-mole.
The human genome encodes about 518 kinases, with virtually every signal transduction process “wired” through a phosphotransfer cascade, suggesting that inhibition of kinase activity can impact cellular physiological responses including cell growth and proliferation. Kinases, therefore, make ideal targets for interrupting the uncontrolled signaling characteristic of cancer cells.
Cancers such as leukemia, some melanomas, and non-small-cell lung cancer (NSCLC) are good kinase inhibitor targets precisely because cancer cells harbor mutations, for example, in the epidermal growth factor receptor (EGFR). But while these mutations confer unique sensitivity to targeted therapies such as EGFR tyrosine kinase inhibitors (EGFR-TKI) like gefitinib and elotinib, acquired resistance to EGFR-TKIs develops after a median of 10 to 14 months, requiring a switch to conventional chemotherapy or in some cases, newer kinase inhibitors that target other kinases.
The remarkable ability of cancers to devise escape hatches from any and all therapies remains the biggest battlefront in the war against this disease. Cancer cell resistance strategies range from serial mutations in drug-targeted receptors such as EGFR, to production of proteins from noncancer cells in their surrounding tissues (stroma) that help fuel cancer to growth and metastases.
At the Mass General Hospital Cancer Center and other institutions and companies, researchers have focused on analyzing mechanisms of targeted drug resistance, with the aim of eventually matching patients with the most effective target therapies possible and the least likely to elicit drug resistance. This, researchers say, will require a range of new diagnostic tools based on a better understanding of the molecular mechanisms underlying the disease.
To that end, Mass General scientists reported that they had modeled acquired resistance to a novel, dual EGFR/Her2-irreversible tyrosine kinase inhibitor, neratinib, in a NSCLC cell culture model.
The scientists performed the experiment, they said, to “anticipate the possibility that patients who respond to irreversible inhibitors will develop secondary resistance to such inhibitors, as has been seen in other, similar settings.” Neratinib (PB272) is a tyrosine kinase inhibitor under investigation for the treatment of breast cancer and other solid tumors by Puma Biotechnology.
The Mass General scientists reported that neratinib could overcome T790M resistance, but only at suprapharmacologic concentrations. Cells that retain phosphorylated EGFR had acquired the secondary mutation T790M.
The investigators further modeled mutations at EGFR C797 (T790M) as a mechanism of resistance to irreversible EGFR inhibitors and showed that, although these mutants are resistant to the irreversible inhibitor, they retain erlotinib sensitivity.
Their findings suggested, they said, that neratinib treatment at maximally tolerated dosing may lead to the emergence of T790M-mediated resistance, whereas treatment with a more potent irreversible inhibitor could yield a resistance mutation at EGFR C797.
At the 2012 ASCO meeting last June, Puma Biotechnology announced results from its ongoing Phase I clinical trial of its PB272 given in combination with paclitaxel and trastuzumab in patients with metastatic Her-2 positive breast cancer.
Alan H. Auerbach, CEO and president of Puma Biotechnology, said, “We are pleased to see such a strong indication of efficacy for PB272. In trials of other anti-Her tyrosine kinase inhibitors given in combination with paclitaxel and trastuzumab, the dose of the tyrosine kinase inhibitor needed to be greatly reduced, by as much as 50%, from the dose typically given as a single agent due to tolerability issues.
“Based on the results of this trial it appears that we may be able to administer PB272, when it is given in combination with paclitaxel and trastuzumab, at doses very close to the dose typically used when PB272 is administered as a single agent. This could position the drug well against other anti-Her tyrosine kinase inhibitors in various settings, including the neoadjuvant setting.”
Resistance mechanisms, as pointed out in a paper by Jeffrey Settleman of Genentech and his team reflect an underlying, recurrent theme. Cancer cells typically express multiple receptor tyrosine kinases (RTKs) that mediate signals converging on key downstream cell-survival effectors such as phosphatidylinositol-3-OH kinase (PI(3)K) and mitogen-activated protein kinase (MAPK).
MAP kinase cascades, for example, are evolutionarily conserved intracellular signal transduction pathways that respond to various extracellular stimuli and control fundamental cellular processes including growth, proliferation, and differentiation.
And scientists are finding that the tumor micro-environment can endow cancer cells with resistance to cancer therapies. David Straussman and his colleagues, in collaboration with Dr. Settleman at Genentech and Todd R. Golub at the Broad Institute in Cambridge found that the stroma, or tissue around cancer cells, mediates resistance of BRAF-mutant melanoma to RAF inhibitors. Using a co-culture system to assay the ability of 23 stromal cell types to influence the innate resistance of 45 cancer cell lines to 35 anticancer drugs, the investigators demonstrated that stroma-mediated resistance is common, particularly to targeted agents.
Proteomic analysis showed that stromal cell secretion of hepatocyte growth factor (HGF) resulted in activation of the HGF receptor MET, and reactivation of MAPK and (PI(3)K)–AKT signaling pathways, resulting in immediate resistance to RAF inhibition.
Studies confirmed stromal cell HGF expression in patients with BRAF-mutant melanoma and showed a correlation between HGF expression by stromal cells and innate resistance to RAF inhibitor treatment. Dual inhibition of RAF and either HGF or MET resulted in reversal of drug resistance, suggesting RAF plus HGF or MET inhibitory combination therapy as a potential therapeutic strategy for BRAF-mutant melanoma, the investigators said.
And, the authors noted, this study indicates that the systematic dissection of interactions between tumors and their micro-environment can uncover important mechanisms underlying drug resistance.
Kinase inhibitor resistance, scientists say, necessitates strategies for developing multiple inhibitors targeting different kinase sites. And overcoming resistance may require an assault on multiple fronts, combining targeted therapies with chemotherapy as studies continue to confirm that cancer cells can evade, by the smallest molecular adjustments, anything thrown at them.