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Multiple Myeloma (MM) is a hematological malignancy resulting in abnormal plasma cells proliferating in the bone marrow. One of the most common treatment strategies for newly diagnosed MM is the combination of induction therapy followed by autologous stem cell transplant (ASCT). Due to the underlying disease and/or the effect of the treatment regimens administered, immunosuppression is a common clinical consequence. It has been shown that ASCT recipients, treated with traditional compounds, have a reduction in the levels of antibody titers to vaccine-preventable diseases such as measles, mumps and rubella (MMR), between one and four years post-transplant. Therefore, re-vaccination is recommended at least two years after ASCT. Therapeutic options have expanded in the past decade, with the introduction of novel agents that have significantly improved MM patient outcomes; however, this may also arguably have new and different implications on a patient's immune system. In this study we sought to analyze the presence of IgG antibodies against MMR in 110 patients with MM post-ASCT. All patients received ASCT between 2014 and 2019 at Mayo Clinic Arizona. Plasma samples were collected approximately 100 days after ASCT (median 92 days) and the antibody titers were tested using the Bio-Rad MMR IgG multiplex flow immunoassay. Sample antibody index values were compared with the assay-specific calibration to determine positivity. For a control population, we utilized the results of fully vaccinated and presumptively immune healthcare workers (HCWs) that were evaluated for the presence of antibodies to measles (n=199), mumps (n=197) and rubella (n=209), using the same method, instrument and laboratory. Overall, 70% of the MM patients were positive for antibodies against measles, compared with 77.4% of HCWs. For mumps, the MM cohort had a positivity of 49.1% versus 84.8% of HCWs. Finally, rubella antibodies were found in 64.5% and 83.7% of MM and HWCs, respectively. Next, we performed testing on serial samples collected from 45 MM patients, comparing the presence of MMR antibodies pre-ASCT (median 130 days) and post-ASCT. The number of patients with positive titers detected pre- and post-transplant was unchanged for each of the three viruses. In summary, our findings do not indicate a significant reduction in relative MMR antibody levels in ASCT recipients. This suggests that earlier re-vaccination is not required post-ASCT in the era of novel MM compounds. However, further validation studies in larger cohorts are necessary prior to considering a change in current vaccination guidelines. Disclosures Braggio: DASA: Consultancy; Bayer: Other: Stock Owner; Acerta Pharma: Research Funding. Fonseca:Janssen: Consultancy; Takeda: Consultancy; Novartis: Consultancy; Bayer: Consultancy; Sanofi: Consultancy; Merck: Consultancy; Pharmacyclics: Consultancy; Amgen: Consultancy; BMS: Consultancy; Celgene: Consultancy; Juno: Consultancy; Kite: Consultancy; Aduro: Consultancy; OncoTracker: Consultancy, Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Consultancy; GSK: Consultancy; AbbVie: Consultancy; Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees.

Abstract Introduction Recent classification of neuroendocrine neoplasms has defined well-differentiated high-grade neuroendocrine tumors (NET G3) as a distinct entity from poorly differentiated neuroendocrine carcinoma. The optimal treatment for NET G3 has not been well-described. This study aimed to evaluate metastatic NET G3 response to different treatment regimens. Materials and Methods This was a retrospective study of patients with NET G3 within the Mayo Clinic database. Patients’ demographics along with treatment characteristics, responses, and survival were assessed. Primary endpoints were progression-free survival (PFS) and overall survival. Secondary endpoints were objective response rate (ORR) and disease control rate (DCR). Results Treatment data was available in 30 patients with median age of 59.5 years at diagnosis. The primary tumor was mostly pancreatic (73.3%). Ki-67 index was ≥55% in 26.7% of cases. Treatments included capecitabine + temozolomide (CAPTEM) (n = 20), lutetium 177 DOTATATE (PRRT; n = 10), Platinum-etoposide (EP; n = 8), FOLFOX (n = 7), and everolimus (n = 2). CAPTEM exhibited ORR 35%, DCR 65%, and median PFS 9.4 months (95% confidence interval, 2.96–16.07). Both EP and FOLFOX showed similar radiographic response rates with ORR 25.0% and 28.6%; however, median PFS durations were quite distinct at 2.94 and 13.04 months, respectively. PRRT had ORR of 20%, DCR of 70%, and median PFS of 9.13 months. Conclusion Among patients with NET G3, CAPTEM was the most commonly used treatment with clinically meaningful efficacy and disease control. FOLFOX or PRRT are other potentially active treatment options. EP has some activity in NET G3, but responses appear to be short-lived. Prospective studies evaluating different treatments effects in patients with NET G3 are needed to determine an optimal treatment strategy. Implications for Practice High-grade well-differentiated neuroendocrine tumors (NET G3) are considered a different entity from low-grade NET and neuroendocrine carcinoma in terms of prognosis and management. The oral combination of capecitabine and temozolomide is considered a good option in the management of metastatic NET G3 and may be preferred. FOLFOX is another systemic option with reasonable efficacy. Similar to other well-differentiated neuroendocrine tumors, peptide receptor radionuclide therapy seems to have some efficacy in these tumors.

e16691 Background: Recent WHO classification of NET has defined high-grade well-differentiated NET (NET G3) as a distinct entity that is clinically different from neuroendocrine carcinoma. The optimal treatment for NET G3 has not been well-described. This study aims to evaluate NET G3 response to different treatment regimens. Methods: This is a retrospective study of NET G3 patients within the Mayo Clinic database (Arizona, Florida, and Minnesota). Patient demographics along with treatment responses and survival were assessed. Primary end points were progression-free survival (PFS) and objective response rate (ORR) by RECIST 1.1 criteria. Results: 71 patients with NET G3 were identified. Systemic treatment data was available in 30 patients who had a median age of 59.5 years at time of diagnosis. The primary tumor was most commonly pancreatic (73.3%). Ki 67 index was ≥55% in 26.7% of cases. 56.7% of cases had > 1 metastatic site. Treatment regimens included: capecitabine+temozolomide (CAPTEM) (n = 20), somatostatin analog (SSA) (n = 14), carboplatin/cisplatin+etoposide (EP) (n = 7), everolimus (n = 2), FOLFOX (n = 7), and lutetium Lu 177 DOTATATE (PRRT) (n = 10). CAPTEM was the most commonly used regimen (10 first-line, and 10 second-line) with ORR of 35%, disease control rate (DCR) of 65%, and median PFS of 9.4 months. The table summarizes the treatment data and responses rates for the various therapies used. Conclusions: Among NET G3 patients treated at Mayo Clinic, CAPTEM was found to be the most commonly used treatment with reasonable efficacy and disease control. Other treatment options include SSA, PRRT, FOLFOX, and EP. Prospective studies evaluating different treatments effects in NET G3 patients are needed to determine an optimal treatment strategy. [Table: see text]

375 Background: Combination FP with P CT have become the standard of care for advanced G/GEJ cancer. Clinical trials in conjunction with practice, have adopted induction FP and P CT for 3-4 months (mos). In other GI malignancies, induction CT followed by maintenance CT (MTC) has been shown to improve patient (pt) outcomes compared to observation, with a decrease in treatment (trmt) related toxicities with induction therapy. However a maintenance approach in G/GEJ cancer has not been investigated in clinical trials. We investigated pt outcomes with metastatic G/GEJ cancer who received continuous induction (CTX) versus induction followed by MTC. Methods: A retrospective analysis of pts with metastatic G/GEJ adenocarcinoma treated with (FP+ P)-based CT between 2007 to 2017 from three centers of a single institution was performed. Metastatic G/GEJ cancer pts who achieved at least stable disease after initial induction trmt were included. Pts were categorized into the CTX group if they received greater than 16 weeks or 8 cycles of combined CT and assigned to the MTC group if they received maintenance FP monotherapy after 8 or less cycles of combined induction CT. Data was extracted from the medical record to determine progression free survival (PFS), overall survival (OS), and toxicities. Results: Sixty-four pts that met criteria and were evaluated, thirty-four received CTX and thirty received MTC. No significant difference in PFS (12.1 vs 8.0 mos p = .72, HR=1.10 95%CI .66-1.83) was observed between the CTX and MTC groups, additionally there was no significant difference in OS. A significant decrease in trmt related toxicities were observed, with a higher proportion of thrombocytopenia (84.8% vs 50.0% p = .004), and grade 3 neuropathy (39.4% vs 13.8% p =.024) in CTX pts (Table). Conclusions: MTC following induction FP/P CT is associated with an improved toxicity profile and appears to be effective compared to CTX in metastatic G/GEJ cancer. Prospective randomized studies confirming its potential benefits compared with continuous induction CT are warranted. [Table: see text]