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A 24-year-old African-American man presented with malaise and low back pain and was found to have acute severe rhabdomyolysis followed by acute hemolysis. Glucose-6-phosphate dehydrogenase (G6PD) deficiency was suspected by the presence of blister cells on peripheral smear and was confirmed by a low enzyme activity assay. Our patient reported playing football, along with upper respiratory infection symptoms, prior to presentation. Extensive infectious and toxicology workup was negative; however, several inflammatory proteins were markedly elevated. We hypothesized the large inflammatory burden led to an increased reactive oxygen radical burden that overwhelmed muscle and erythrocyte reducing power. Severe rhabdomyolysis in G6PD deficiency is not a common presentation because skeletal muscles are more resistant to oxidative damage compared to red blood cells. Our case adds to the few existing reports of myolysis in the setting of G6PD deficiency.

e12538 Background: CDK inhibitors (CDKi), in combination with aromatase inhibitors (AI), are approved for the treatment of hormone receptor positive (HR+) Her2 negative metastatic breast cancer (MBC). The effectiveness of continuing CDKi beyond first disease progression is not known. This study evaluated real world evidence and assessed the impact of continuation of CDKi beyond first disease progression in combination with endocrine therapy. Methods: This is a retrospective, single institution review of HR+ MBC patients treated with CDKi from 2015-2018 who continued CDKi after initial progression. The primary outcome was progression-free survival (PFS) beyond first disease progression, as assessed by the clinician based on radiological and/or clinical criteria. Overall survival (OS) – defined as date of initial CDKi treatment to date of death or last follow up – was a secondary outcome. Results: 30 women with HR+/HER2- MBC, median age 47.5 years (range: 31 – 81), sequentially continued on CDKi beyond first progression were identified from a database of patients treated with Palbociclib. Median and average follow up times on CDKi were 27.18 and 24.53 months, respectively. Initial endocrine/CDKi regimen received included: palbociclib (PA)/letrozole (LTZ) [67%], PA/fulvestrant (FULV) [23%], and PA/other AI [10%]. Prescribed combinations beyond 1st progression were: PA/FULV [56.7%], PA/LTZ [16.7%], and PA/other AI [20%], abemaciclib plus LTZ or FULV [6%]. As of 1/31/2019, 25 patients (83.3%) were still alive, and 19 (63%) had undergone a second progression on CDKi. The estimated median PFS for the entire duration while on CDKi was 23.5 months (95% CI 12.8 – 27.8), of which 11.8 months (95% CI 5.34 – 13.13) was the median PFS beyond first progression. The estimated median OS was 45.4 months. Conclusions: Among a small cohort of HR+ MBC patients, in a non-clinical trial setting, continuation of palbociclib plus endocrine therapy beyond first progression was associated with a median PFS of approximately 11 months. Formal clinical evaluation of continuation of CDK inhibitor plus endocrine therapy beyond first progression is warranted.

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