There are two major subtypes of lung cancer: Non-Small Cell Lung Cancer (NSCLC), which accounts for 85% of all cases, and Small Cell Lung Cancer (SMLC). About 60% of NSCLC are unresectable at diagnosis, hence, the poor prognosis – ten to twelve months survival when treated with platinum-based chemotherapy. Treatment options are evaluated based on the histologic subtype and the presence of mutations to determine the the best combination of molecular therapies for treatment. Ten to twenty percent of patients with NSCLC have a mutated epidermal growth factor receptor, most commonly. a deletion in the in-frame of exon 19 (around amino acid 747 to 752) or a L858R point mutation of exon 21. On June 1, 2016, the FDA approved the first blood test (liquid biopsy) companion diagnostic to determine whether these mutations are present.
Epidermal growth factor receptor (EGFR) is a tyrosine kinase receptor (RTKs) that is expressed on the surface of cells and transduces signals from the cell’s exterior to the cytoplasm. Ligand binding to the N-terminal ectodomain of the cognate receptor causes the receptor to dimerize and activate each monomer’s kinase to phosphorylate partner tyrosines on the C-terminal cytoplasmic domain. Phosphotyrosines allow for emission of growth signals to target proteins within the cell. Mutations that cause an overexpression of EGFR can lead to tumor promoting properties like proliferation, anti-apoptotic characteristics, angiogenesis and migration because of downstream signaling that activates signal transduction cascades.
EGFR inhibitors that have been used to target the overexpression of EGFR protein, like gefitinib, erlotinib, afatinib, lead to drug resistance; most commonly by the T790M resistance mutation. The T790M resistance occurs due to an amino acid substitution of methionine for threonine at the 790 position at exon 20. An Oncology Drug Advisory Committee voted 12 to 1 against an accelerated approval of Clovis Oncology’s rociletinib for patients that have an EGFR mutation positive NSCLC that were previously treated with an EGFR targeted therapy and tested positive for the T790M resistance mutation.
Rociletinib is Clovis’ novel, oral and irreversible mutant-selective inhibitor of EGFR. Rociletinib is a third generation EGFR inhibitor for the T790M mutation. Rociletinib blocks both the activating EGFR mutations and the T790M (which confers resistance to first and second generation TKI’s), allowing it to suppress the growth of T790M positive cells. This is important because tumors that progress following first-and second-line EGFR TKI treatment are heterogeneous, that is, comprised of clones of cells that are both T790M positive and negative (wild-type).
AstraZeneca’s Tagrisso (osimertinib) received accelerated approval for patients with the T790M in November of 2015 with an overall objective response rate of 59%.
In recommending against accelerated approval, the rociletinib panel said existing data on rociletinib did not adequately characterize its benefit-risk profile over current treatments, especially Tagrisso, and also expressed uncertainty about the proposed dose. In clinical trials, patients that were positive for EGFR-T790M mutation in metastatic NSCLC were given dosages ranging from 500 mg to 1000mg twice daily – the objective response rate across all doses was 30.2% with a mediation duration of response of 8.9 months. More than half of the patients taking any of the doses of rociletinib experienced an adverse event. Nearly half experienced a serious adverse event and 17% of the patients in the 625 mg arm died due to an adverse event. Evidence of hyperglycemia and QT prolongation–lethal heart arrhythmia–were the two leading causes of dose modification.