Small Cell Lung Cancer: Can Recent Advances in Biology and Molecular Biology Be Translated into Improved Outcomes?

Despite the paucity of therapeutic advances in SCLC, considerable progress in understanding the biology, molecular biology, model systems and potential therapeutic targets has been made. Studies of early lung and neuroendrocrine cell development models have provided insights into the cell of origin for SCLC. New GEMMs have illustrated the universal importance of TP53 and RB1 gene mutations in the pathogenesis and the potential role of additional genetic changes as well as changes in transcription factor expression. PDXs and CDXs provide new means for preclinical testing of new therapies. Molecular studies have identified the high mutation burden found in SCLC and have identified differences between SCLC, carcinoids and large cell neuroendocrine tumors. Potential therapeutic targets including EZH2, PARP, CDK1, MCL1, BCL2, BIM, SHH (Sonic Hedgehog), WNT, NOTCH1, Aurora Kinase, FGFR, PIK3CA, RET, THZ1, JAK-STAT, FAK, CXCR4, PD-L1, Fuc-GM1, CD56 and CD47. Ongoing and future clinical trials have to show which of these candidates can be translated into an effective targeted therapy. Thus, the future of improving outcomes for SCLC patients appears promising but there are still a number of unanswered questions which need to be addressed in the future and these are outlined below. Small Cell Lung Cancer Major Questions of Translational Relevance What are the mechanisms underlying the universal development of chemoresistance? SCLC is in nearly every case very sensitive to platinum-etoposide chemotherapy with dramatic clinically beneficial responses. However, in nearly every case the tumors become resistant to this chemotherapy. What is the mechanism of this resistance, are there ways to avoid it, and what are additional therapies that could kill such resistant tumor cells? What are the mechanisms of highly metastatic behavior in SCLC? SCLC is in nearly every case highly metastatic from the time of clinical diagnosis. What are the most important mechanisms responsible for the metastatic behavior and can these be therapeutically targeted? A subset of this question, is that SCLC appear to be much enriched in cancer stem cells (“tumor initiating cells”, TICs) and does targeting stem cell signaling pathways provide effective therapy for primary and metastatic sites of SCLC? Are there therapeutic targets of “acquired vulnerability” associated with RB1 or TP53 mutations in SCLC? Tumor suppressor genes RB1 and TP53 abnormalities are essentially universal in SCLCs. Are there acquired vulnerabilities associated with changes in these two genes that can be therapeutically targeted? Do the many other mutations occurring in human SCLC (but not in GEMMs) provide therapeutic targets – “acquired vulnerabilities for human SCLC?” Current evidence indicates that human SCLCs have many other genetic (mutations) and epigenetic changes besides those involving the key oncogenic drivers (such as TP53, RB1, LMYC, NFIB). By contrast, mouse GEMM SCLC have very few additional changes. Presumable the differences in mutation rates result from exposure to cigarette carcinogens in humans but not in mice. Do the other mutations in human SCLC provide acquired vulnerabilities of therapeutic vulnerabilities, do any these represent “synthetic lethalities” with the main driver oncogene changes, and are there several vulnerabilities common across multiple SCLCs or are these vulnerabilities “private” and found in only individual tumors? Can we develop therapeutic strategies targeting important SCLC driver transcription factors? Several transcription factors appear to be very important in the growth and survival of SCLC including ASCL1, NeuroD1, myc family members, and SOX2). Are there therapeutic strategies that can be directed against these key transcription factors and do they provide quantitatively multiple logs of tumor cell kill to allow development of curative strategies? What are the important differences in human preclinical models of SCLC that influence discovery and validation of new therapies? Currently there are several types of human preclinical models of SCLC including SCLC lines, SCLC cell line xenografts (CDXs), patient derived SCLC xenografts (PDXs), and circulating SCLC tumor cells isolated from peripheral blood of SCLC patients. While these all share common genetic abnormalities found in SCLC (such as TP53, RB1, myc family members), we need to know what are the molecular differences between these different models and SCLC tumor samples and whether preclinical therapeutic responses are similar or different between these models. Can we continue to use all of the models for development of new therapies or do we need to only use one type of preclinical model? Are the differences between human SCLC and GEMM of SCLC that influence the discovery and validation of new therapies? Genetically engineered mouse models of lung cancer (GEMMs) appear to be very similar to human SCLC. However, it appears that mouse GEMM SCLC do not appear to be sensitive to platin-etoposide chemotherapy. What are the differences between human and the GEMM models of SCLC that explain this discrepancy? Are there important differences between human and GEMM preclinical models that could influence the discovery and validation of new SCLC therapies? What are the biologic reasons for SCLCs expressing ASCL1 vs NeuroD1 as a lineage oncogene? The majority of human SCLCs have ASCL1 as the major neuroendocrine lineage driver gene. However, a subset of human SCLCs express high levels of NeuroD1 and not ASCL1. Currently, there is no evidence that mouse lung neuroendocrine cells can express Neuro D1. This raises the question of whether small cell cancers predominantly expressing NeuroD1 arise in the human lung or in some other primary site. Do genetic abnormalities in histologically normal epithelium of SCLC patients provide diagnostic and chemopreventative targets? There appear to be a much greater frequency of genetic abnormalities in the histologically normal epithelium of patients with SCLC compared to those with NSCLC. Does this information provide ways for early diagnosis or implementation of chemoprevention strategies for SCLC using these genetic alterations as molecular biomarkers? Most recently the US National Cancer Institute released a Request for Application (RFA) (PAR-16-049, PAR-16-050, PAR-16-051) for grants specifically focusing on SCLC, and hopefully through these grants many of the above questions will be addressed.