One of the earliest success stories illustrating pathway-driven therapeutics is with CML. The hallmark chromosomal abnormality of this disease is the translocation of chromosomes 9 and 22 that ultimately produces the fusion gene BCR-ABL. This discovery in 1960 eventually led to the development of the targeted tyrosine-kinase inhibitor (TKI) imatinib and its subsequent Food and Drug Administration (FDA) approval for treatment of CML in 2001.⁶ The International Randomized Study of Interferon and STI571 (IRIS) trial began enrollment in 2000 and compared imatinib with interferon and low-dose cytarabine, which was the previous standard of care for newly diagnosed patients with chronic-phase CML. All efficacy endpoints favored imatinib, including complete cytogenetic response of 76.2% with imatinib compared with 14.5% with interferon (p <0.001).⁷ Overall survival (OS) after 60 months of follow-up was 89% with imatinib.⁸ This example is just one of many where a once fatal disease can now be considered more akin to a chronic disease, requiring a daily medication and regular physician follow-up, similar to hypertension or diabetes. Drug development has also kept pace with these advances and now several other agents, including dasatinib, nilotinib, bosutinib, and ponatinib, have joined imatinib as treatment options for CML.

The idea of changing treatment focus from a disease-based model to a pathway-driven model is also evolving. Human epidermal
growth factor receptor 2 (HER2) is a transmembrane receptor tyrosine kinase that is overexpressed or amplified in up to 25% of breast cancers. Trastuzumab is a humanized monoclonal antibody directed against HER2 and demonstrated improved response rates (RR) and time to disease progression in patients with metastatic HER2 positive breast cancer and improved disease-free survival (DFS) and OS in HER2-positive breast cancer patients treated with adjuvant trastuzumab.⁹ Several additional agents are now available to target the HER2 pathway and vary in their pharmacology and mechanism of action. Lapatinib is an oral TKI directed against HER2 and the epidermal growth factor receptor (EGFR), pertuzumab is a humanized monoclonal antibody that binds at a different location than trastuzumab and inhibits the dimerization and subsequent activation of HER2 signaling, and ado-trastuzumab emtansine is an antibody-drug conjugate that targets HER2-positive cells and then releases the cytotoxic antimitotic agent emtansine through liposomal degradation of the linking compound. All of these agents illustrate the progress and pharmacologic diversity of pathway-directed therapy and remain as standard of care options for HER2-positive breast cancer in either the adjuvant and/or metastatic settings.¹⁰ HER2 expression is not limited to breast cancer, however. Though less common, HER2 expression is seen in numerous solid tumors including bladder, gastric, prostate and non-small-cell lung cancer with varying degrees of incidence depending on the method of detection. Based on results from a large, open-label phase III randomized, international trial of 594 patients with gastric or gastroesophageal junction cancer expressing HER2 by either immunohistochemistry or gene amplification by fluorescence in situ hybridization, trastuzumab is also approved for treatment of metastatic gastric or gastroesophageal junction adenocarcinoma that expresses HER2. Patients randomized to chemotherapy in combination with trastuzumab had a median OS of 13.8 months compared with 11.1 months in the patients receiving chemotherapy alone (hazard ratio [HR], 0.74; 0.60 to 0.91, p = 0.0046).¹¹ Numerous examples also support that pathway-directed therapy will cross the boundaries of disease sites and that tumor genetics will become one of the biggest determining factors for treatment.

Simple expression of the drug target does not always translate into desired clinical outcomes though. Cetuximab and panitumumab are monoclonal antibodies directed against EGFR; however, it was found that colorectal cancer (CRC) patients who did not have detectable EGFR still experienced responses to these agents similar in extent to EGFR-positive patients. Kirsten rat sarcoma viral oncogene (KRAS) is a downstream effector of the EGFR pathway. Ligand binding to EGFR on the cell surface activates pathway signaling through the KRAS-RAF-mitogen-activated
protein kinase (MAPK) pathway, which is thought to control cell growth, differentiation, and apoptosis.¹² Eventually it was found that CRC patients with a KRAS mutation did not derive benefit from cetuximab or panitumumab. The RR in CRC receiving either cetuximab or panitumumab who were KRAS wild type was 10% to 40% compared with near zero percent in those with KRAS mutations.¹³ This finding was the result of a retrospective analysis of small group of patients and was confirmed in large, prospective trials. Additionally, it underscores the importance of tissue collection for biomarker assessment in trials with novel therapeutics. A recent clinical trial genomic analysis suggests that mutations in NRAS may also have value in predicting the utility of EGFR antibody therapy in colorectal cancer. Although the predictive value of KRAS mutation status in colorectal cancer has been well established in clinical trials, the role of KRAS in lung cancer and other malignancies is less well elucidated. Lung cancers harboring KRAS mutations have been shown to have less clinical benefit from the EGFR-targeted erlotinib in some trials, although this has not consistently been the case across all trials. Additionally, lung cancer KRAS mutation status does not appear to reproducibly predict clinical benefit from the EGFR-targeted monoclonal antibodies, as is the case in colorectal cancer.¹⁴ Unlike the HER2 example discussed previously, the clinical application of some genetic mutations will differ between tissue of origin.

Deeper investigations and understandings of mutations driving oncogenic pathways can also elucidate mechanisms of resistance and practical therapeutic strategies for treatment and prevention. Approximately half of all cutaneous melanomas carry mutations in BRAF, with the most common being the V600E mutation. Vemurafenib is a TKI directed against mutated BRAF that demonstrated improvements in both progression-free survival (PFS) and OS when compared with the cytotoxic agent dacarbazine in previously untreated patients with metastatic melanoma carrying the BRAF V600E mutation. Vemurafenib demonstrated a 63% relative reduction in the risk of death compared with dacarbazine (p <0.001) along with a higher response rate (48% compared with 5% for dacarbazine).¹⁵ Based on these results, vemurafenib was the first BRAF targeted TKI approved by the FDA and was soon joined by dabrafenib. Although dramatic responses to these agents have been observed, relapse almost universally occurs after a median of 6 to 8 months. Activating BRAF mutations, like V600E, result in uncontrolled activity of the MAPK pathway through activation of the downstream kinase MEK, which when phosphorylated, subsequently activates extracellular signal-regulated kinase (ERK), which ultimately translocates to the cell nucleus, resulting in cell proliferation and survival (Fig. 16.1).¹⁶ An assessment of serial biopsies from patients treated with vemurafenib suggested numerous mechanisms for acquired resistance, including the appearance of secondary mutations in MEK.¹⁷ This finding supports the clinical rationale for using combination therapy with a BRAF and a MEK inhibitor. The combination of dabrafenib (BRAF inhibitor) and trametinib (MEK inhibitor) was assessed in 247 metastatic melanoma patients with BRAF V600 mutations compared with dabrafenib alone. Median PFS was 9.4 months in the combination group compared with 5.8 months in the patients who received single agent therapy (HR, 0.39; 0.25 to 0.62, p <0.001). A complete or partial response was also higher in the combination therapy group (76% compared with 54%, p = 0.03). The occurrence of cutaneous squamous cell carcinoma, a known side effect of single-agent BRAF inhibitor therapy due to paradoxical activation of RAF in nonmutated cells, was also decreased in the combination therapy group (7% compared with 19%, p = 0.09), further supporting the evidence of downstream inhibition.¹⁸ Although combination therapy does prolong the time to disease progression, resistance still occurs in patients through a variety of mechanisms. Utilization of sequential biopsies and a genetic assessment will help to inform rationale combination and sequential pathway-driven therapy trials that will ultimately aid in better understanding and mitigation of common mechanism of resistance.