Coston, Tucker

Abstract Introduction Pathogenic mutations in POLE/POLD1 lead to decreased fidelity of DNA replication, resulting in a high tumor mutational burden (TMB-H), defined as TMB ≥ 10 mut/Mb, independent of deficient mismatch repair (dMMR) and microsatellite instability high (MSI-H) status. Methods De-identified records of patients with colorectal cancer (CRC) profiled with the Tempus xT assay (DNA-seq of 595-648 genes at 500×) were identified from the Tempus Database. Results Among 9136 CRC samples profiled, the frequency of POLE/POLD1 genomic alterations was 2.4% (n = 217). Copy number loss was the most common genomic alteration (64%, n = 138) of POLE/POLD1, followed by copy number amplifications (18%, n = 40) and short variant mutations (18%, n = 39). The POLE/POLD1 mutated group presented with a higher frequency of TMB-H phenotype relative to wild type (WT; 22% vs. 9%, P < .001), with a median TMB of 127 mut/Mb in the TMB-H POLE/POLD1 subset. The TMB showed a dramatic contrast between POLE/POLD1 short variant mutations as compared to the group with copy number alterations, with a TMB of 159 mut/Mb vs 15 mut/Mb, respectively. Thus, the short variant mutations represented the so-called ultra-hypermutated phenotype. The POLE/POLD1 mutated group, as compared to WT, exhibited a higher rate of coexisting mutations, including APC, ALK, ATM, BRCA2, and RET mutations. Conclusion Patients with POLE/POLD1 mutations exhibited significant differences across immunological markers (ie, TMB, MMR, and MSI-H) and molecular co-alterations. Those with short variant mutations represented 18% of the POLE/POLD1 cohort and 0.4% of the total cohort examined. This group of patients had a median TMB of 159 mut/Mb (range 34-488), representing the ultra-hypermutated phenotype. This group of patients is important to identify given the potential for exceptional response to immune checkpoint inhibitors.

592 Background: The introduction of peptide receptor radionuclide therapy (PRRT) using Lutetium-177 (177Lu) DOTATATE has led to a paradigm shift in the treatment of neuroendocrine tumors (NETs). Hematological toxicity is a well-recognized adverse effect (AE) of 177Lu DOTATATE. Most cytopenias are transient, with an estimated incidence of 10-25%, typically mild to moderate in severity (grade G1-2). The most dreaded AE of PRRT is therapy-related myeloid neoplasm (t-MN), with an estimated 2-8% incidence and a higher incidence reported in patients who also received prior chemotherapy. We sought to evaluate the hematological safety of 177Lu PRRT in the setting of NETs with extensive (i.e., innumerable) bone metastases. Methods: We retrospectively reviewed the medical records of all Mayo Clinic patients (pts) with extensive/innumerable osseous metastases, defined as >50% skeletal involvement by positron emission tomography (PET) DOTATATE, who were treated with 177Lu DOTATATE. Pts characteristics and laboratory results were collected before, during, and after 177Lu DOTATATE treatment. Hematotoxicity was graded according to the NCI-CTCAE v5. Results: Out of 27 pts, 13(48%) developed cytopenia(s) of any grade after treatment with one or more cycles of 177Lu DOTATATE. In total, there were 13(48%) pts with anemia, 13(48%) with thrombocytopenia, and 6(22%) with neutropenia. Six (22%) pts had severe hematological toxicity (G3-4). Twelve months post-PRRT, 7(26%) pts continued to experience cytopenia(s) of any grade, of whom 4(15%) had G3-4 hematotoxicity. One pt developed t-MN, and two pts had myelophthisis on bone marrow biopsy from infiltrating NET. 16(59%) pts received cytotoxic chemotherapy before PRRT, and 7(25.9%) developed subsequent cytopenia(s). Overall, 15(55%) pts completed four cycles, while 3 pts didn’t complete four cycles of 177Lu DOTATATE due to hematotoxicity. Conclusions: Pts with advanced NETs and extensive/innumerable bone metastases treated with PRRT could be at higher risk for myelosuppression than historical controls. Our study highlights the importance of carefully monitoring and assessing hematological parameters in pts being considered for 177Lu DOTATATE with extensive/innumerable bone metastasis. [Table: see text]

PURPOSE Pancreatic cancer (PC) carries a poor prognosis with high rates of unresectable/metastatic disease at diagnosis, recurrence after resection, and few systemic therapy options. Deficient mismatch repair (dMMR)/high microsatellite instability (MSI-H) PCs demonstrated uncharacteristically poor outcomes in KEYNOTE-158, evaluating pembrolizumab in MSI-H solid tumors. Our study aggregates the Mayo Clinic experience with dMMR/MSI-H PCs, characterizing the clinical, molecular, and treatment response patterns with a focus on response to immune checkpoint inhibitors (ICIs). METHODS Retrospective data were collected from the electronic medical record from December 2009 to February 2023. Patients were included if they had a pathologically confirmed pancreatic malignancy and had (1) deficient expression of mismatch repair (MMR) proteins by tumor immunohistochemistry, (2) pathogenic mutation of MMR genes on genomic sequencing, and/or (3) MSI-H by polymerase chain reaction. RESULTS Thirty-two patients were identified for inclusion, with all stages of disease represented. Sixteen of these patients underwent surgery or chemoradiotherapy. Of these patients, uncharacteristically favorable responses were seen, with a recurrence rate of only 19% (n = 3) despite a median follow-up of 25 months. In the palliative setting, excellent responses to ICI were seen, with overall response rate (ORR) of 75% (20% complete response). Median time to disease progression was not reached. Response rates to cytotoxic chemotherapy in the palliative setting were poor, with 30% ORR and median time to progression of 4 months. We observed a high rate of discrepancy between MMR and MSI testing methods, representing 19% of the entire cohort and 26% of evaluable cases. CONCLUSION Our data argue for the preferential use of ICI over cytotoxic chemotherapy in any patient with dMMR/MSI-H PC requiring systemic therapy, including in the metastatic and adjuvant/neoadjuvant settings.

641 Background: Tumors of neuroendocrine origin are a rare, heterogenous group of neoplasms. Neuroendocrine neoplasms (NENs) are categorized by site of origin, differentiation status, and by grade (Ki-67 expression and/or mitotic rate), with prognostic variability accordingly. These tumors frequently metastasize to bone, with reported incidence between 6-12% by older SSTR imaging. Our study evaluates patients with well-differentiated tumors of neuroendocrine origin to determine the incidence of osseous metastases when evaluated with higher-sensitivity Ga68 DOTATATE PET scans. The study characterizes the clinical features. Methods: This study was performed at a single, 3-site, US tertiary-care institution. IRB approval was obtained. An automated data extraction tool was used to mine the electronic medical record by searching all positron emission tomography (PET) studies for keywords. Identified scans had to include a combination of the following keywords: “Dotatate” AND “met*” or “lesion” AND “bone” or “osse*” or “skel*”. The individual medical records from the generated report were reviewed to include only patients with 1) well-differentiated NETs of GI and pancreatic origin, lung carcinoid, paraganglioma/pheochromocytoma, or other/unknown primary site, and 2) patients with confirmed osseous metastatic disease. Patient data was entered into a database and evaluated in aggregate. Results: 1,948 PET scans of 1,473 patients were extracted from the EMR, from which 424 patients were identified for inclusion; scans were performed between 5/2018 and 5/2021. Calculated incidence of bone metastasis by Ga68 DOTATATE PET was 28.8%. Median age of included population was 61 years (range 14-92), 49.5% being male. Site of origin was 47.2% bowel NET, 18.9% pancreatic NET, 10.8% lung carcinoid, 10.6% paraganglioma/pheochromocytoma, 2.1% other site, and 10.4% unknown primary. Majority of patients were asymptomatic (64.0%), had sclerotic appearance (76.7%), Krenning 4 (71.4%), and >3 sites (68.3%) of osseous disease. 94.6% of the population had disease of the axial skeleton; 65.6% appendicular. Only 57 patients (13.4%) with osseous disease suffered a fracture, despite metastases at high-risk sites. Fracture occurred at disproportionately low rates in NETs originating in bowel (22.8% of fractures), with proportionately higher rates among pancreatic NETS and paragangliomas/pheochromocytomas (31.6% and 22.8%, respectively). Fractures occurred at proportionately higher rates in higher-grade disease compared to low-grade. Conclusions: Osseous metastatic disease in well-differentiated NENs is evident at much higher rates when imaging with Ga68 DOTATATE PET compared with previously reported data. Nevertheless, fracture occurred at a low rate, suggesting that these patients are at a relatively low risk for skeletal-related events.

199 Background: The progression of neoplasia in colorectal cancer (CRC) has been well characterized, with the evolution typically involving mutations in APC, KRAS, and p53, among others. POLE/POLD1 genes encode for proteins essential in enzymatic DNA polymerase function, yet POLE/POLD1 mutations in tumors are poorly characterized. Pathogenic mutations in POLE/POLD1 lead to decreased fidelity of DNA replication, resulting in a high tumor mutational burden (TMB-H) independent of deficient mismatch repair (dMMR) and high microsatellite instability (MSI-H). Studies have shown associations between this hypermutated phenotype and susceptibility to immune checkpoint inhibition, which may be attributable to neoantigen creation. As such, these tumors are a clinically relevant phenotype requiring further investigation. Here, we characterized POLE/POLD1 alterations in a large, real-world cohort of patients with CRC. Methods: We retrospectively analyzed de-identified records of 9,136 primary CRC patients profiled with the Tempus xT assay. The assay includes DNA-seq of 595-648 genes at 500x to evaluate single-nucleotide variants, insertions/deletions, and copy number changes. Immunological markers analyzed included tumor mutational burden (TMB), MSI, and dMMR. MSI-H was determined by the assay through assessment of 239 loci. dMMR was determined by immunohistochemistry. Results: A total of 217 patient tumors harbored somatic POLE/POLD1 mutations (n = 203 POLE, n = 13 POLD1, and n = 1 POLE and POLD1; none germline), with no differences noted in baseline demographics including gender and age. Among the POLE/POLD1-mutant cohort, POLE copy number losses accounted for most mutations (n = 127, 59%), with short variants and copy number amplifications accounting for 35% (n = 76). The remainder of tumors exhibited POLD1 copy number changes (n = 14), including one with both POLE and POLD1 copy number loss. POLE/POLD1-mutated tumors presented with a lower frequency of MSI-H compared to wild-type (1.8% vs 6.1%, P= 0.018). Conversely, there was a higher frequency of TMB-high cases ( > 10 mut/Mb) for POLE/POLD1-mutated compared to wild-type tumors (22% vs 9.9%, P< 0.001). Differences between POLE/POLD1-mutated and wild-type tumors were also observed among many co-mutated genes, including APC (80% vs 65%, P< 0.001), ALK (31% vs 11%, P< 0.001), ATM (12% vs 3.6%, P< 0.001), BRCA2 (12% vs. 3.2%), and RET (24% vs 8.9%, P< 0.001). Conclusions: Patients with POLE/POLD1 mutations exhibited significant differences across immunological markers and molecular co-alterations. POLE/POLD1 mutations in CRC have been previously described to present with a hypermutated phenotype; however, 78% of tumors exhibited low TMB despite POLE/POLD1 mutations. Thus, these results have identified POLE/POLD1-mutated tumors as a unique genomic subpopulation, and further studies are needed to better characterize POLE/POLD1 mutations in CRC.

10588 Background: Detection of pathogenic germline variants (PGVs) has implications for cancer screening, prognostication, treatment selection, clinical trial enrollment and family testing. Published guidelines provide indications for PGV testing, determined by clinical and demographic factors, but these leave a significant number of cases missed by guideline-directed testing (incremental PGV). We have previously shown that 50% of patients with PVGs would not have been detected with guideline-based testing alone. In this project, we aimed to determine the feasibility and generalizability of this approach within a more racially/ethnically diverse cohort from a community cancer practice. Methods: We completed a prospective study of proactive germline genetic sequencing among patients with solid tumor malignancies at a community-based oncology practice in downtown Jacksonville, FL between June 2020, and September 2021. The patients were unselected for cancer type, stage, family history, race/ethnicity, and age. PGVs identified using an 84-gene NGS platform were stratified by penetrance. National Comprehensive Cancer Networks (NCCN) guidelines were used to determine incremental PGV rates. Results: 221 patients were enrolled, median age 63.0 years, 21.7% male. 56.6% of patients were white (non-Hispanic), 32.6% were black/African American, 5.4% were Hispanic. 39.8% of patients were commercially insured, 52.4% insured by Medicare/Medicaid and 2.7% uninsured. The cohort was comprised predominantly of breast cancer (61.5%), lung cancer (10.4%) and colon cancer (7.2%) patients. A total of 27 PGVs were identified in 23 patients. We found no significant difference in the rate of PGVs based on race/ethnicity. The overall rate of variants of uncertain significance (VUS) was numerically higher in black/African American patients (61.1%) compared with other ancestry (45%). 13 of 23 (56.5%) patients with detected PGVs did not meet NCCN guidelines for germline genetic screening (incremental PGVs). Conclusions: This prospective study found that universal multi-gene panel testing among patients from a racially/ethnically diverse cohort from a community cancer practice was associated with an increased detection of heritable variants with treatment implications over the predicted yield of targeted testing based on guidelines. This study is the first to evaluate this approach in an exclusively community-based practice. Clinical trial information: NCT04456140.

4145 Background: Pancreatic ductal adenocarcinoma (PDAC) is the 4th leading cause of cancer death. Outcomes remain poor, due to irresectability at diagnosis for many and sub-optimal responses to systemic therapy. Cytotoxic chemotherapy remains the standard of care. High microsatellite instability (MSI-H) predicts response to immune checkpoint inhibition (ICI) in many cancers. Detecting high MSI is rare in PDAC (incidence <2%), but case reports demonstrate potential therapeutic benefit with ICI. Here, we present multi-institutional data to characterize the clinical behavior of MSI-H PDAC, with special attention to response to ICI. Methods: Cases of MSI-H PDAC were obtained by reviewing data of all PDAC patients from our tertiary cancer center who had undergone genomic sequencing by one commercially available platform. The resulting cohort was supplemented with MSI-H PDAC cases identified by GI oncology specialists at multiple institutions. De-identified patient data were compiled and analyzed. Results: 15 MSI-H PDAC patients were identified. 20% had stage II disease at diagnosis, 27% stage III, and 53% stage IV. 73% of patients received ICI (n=11); 40% as 1st line and 33% as 2nd line. These patients demonstrated 100% overall response rate; 45% complete response (1 pathologic CR, 4 radiographic CR) and 55% partial response. No patient that received ICI had lost response or died after a median follow-up of 18 months (range 6-89 mos). 1 patient had oligoprogression of a single hepatic lesion after 7 mos that was then irradiated; this patient retained radiographic CR for 17 subsequent mos (ongoing). In this cohort, we observe poor responses to cytotoxic chemotherapy. In total, 12 regimens were trialed among 9 patients. Overall response rate was 0%. 42% achieved disease stability, with median duration of response of 2 mos; only 2 cases maintained disease stability for >5 mos. 4 patients did not receive ICI; all patients died, with a median survival of 7.5 mos. Conclusions: MSI-H PDAC represents a rare but important subtype of PDAC with unique clinical behavior. Given its rarity, large-scale analyses and trials are unlikely to be performed, making case series such as ours crucial. In our cohort, we observe impressive, durable responses to ICI, along with very poor responses to cytotoxic chemotherapy. Our data argues for consideration of ICI in any patient presenting with MSI-H PDAC, including in the first-line and neoadjuvant settings.

AbstractHypomethylating agents (HMA) are currently the only FDA approved therapy for patients with chronic myelomonocytic leukemia (CMML). In the current retrospective study, we assessed response rates as adjudicated by the IWG (International Working Group) MDS (myelodysplastic syndrome) and MDS/MPN myeloproliferative neoplasm overlap syndrome response criteria, in 121 CMML patients treated with Azacitidine (AZA, n = 56) and Decitabine (DAC, n = 65). The overall response rates were 41% by the IWG MDS (AZA‐ 45%, DAC‐39%), and 56% by the IWG MDS/MPN (AZA‐56%, DAC‐58%) response criteria, with CR (complete remission) rates of <20% for both agents, by both criteria. There were no significant differences in response rates between proliferative and dysplastic CMML. Moreover, 29% of CMML patients in a CR with HMA progressed to AML (blast transformation), underscoring the limited impact of these agents on disease biology. Progression after HMA response was associated with a median overall‐survival (OS) of 8 months, while median OS in patients with primary HMA failure was 4 months. Lower serum LDH levels (<250 Units/L) were associated with HMA responses by both criteria; while ASXL1 and TET2 mutational status had no impact. HMA treated patients had a longer median OS (31 vs 18 months; P = .01), in comparison to those treated with conventional care regimens (excluding observation only patients), without any differences between AZA vs DAC (P = .37). In conclusion, this study highlights the inadequacies of HMA therapy in CMML, retrospectively validates the IWG MDS/MPN response criteria and underscores the need for newer, rationally derived therapies.