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e16017 Background: The primary objective of the study is to assess the feasibility of obtaining combined FDG and NaF PET/CT in detecting soft-tissue and bone metastatic disease in mGU pts treated with CaboNivo+/-Ipi. Methods: Pts in this single-arm, multicenter, phase I trial had FDG PET/CT followed by NaF PET/CT within 1 hour, at baseline and 8 weeks. The number and location of metastatic lesions on FDG PET/CT and on NaF PET/CT were captured. Lesions were considered positive when there was focal abnormal radiotracer uptake with CT correlation. We analyzed soft tissue and bone lesions distribution and the concordance of baseline metastatic bone disease. Results: From 7/14/15 to 9/9/16, 27 pts (urothelial carcinoma N = 11, bladder squamous cell carcinoma N = 2, sarcomatoid renal cancer N = 1, sertoli cell N = 1, germ cell tumor N = 4, trophoblastic testicular cancer N = 1, urachal N = 4, castrate-resistant prostate cancer N = 2, prostate neuroendocrine N = 1) had scans. Median age was 55 (range 35-75 yr), 24 (89%) were male. Pts completed the dual PET/CTs on the same day (imaging time 3-4 hours) without complications. On FDG, the distribution of 340 lesions were lymph node 41%, bone 35%, lung 18%, liver 11%, soft tissue 4%, kidney 0.6%, adrenal 0.3%, spleen 0.3%, pancreas 0.3%. On NaF, the distribution of 130 lesions was spine 33%, rib 16%, pelvis 13%, femur 10%, scapula 9%, sacrum 8%, skull 5%, humerus 4%, sternum 3%, manubrium 2%, clavicle 2%. Of the 130 lesions on FDG/NaF PET/CT, 38% were not seen on initial FDG. The combination of the two tracers did not affect the quality of the PET/CTs given distinct radiotracer uptake in the anatomical locations with no distortions. Residual FDG activity on combined FDG-NaF images required background intensity to be adjusted to delineate focal radiotracer uptake. There was no difficulty in identifying metastatic lesions, and no instances of prior FDG uptake affecting the determination of metastatic disease. Conclusions: Combination FDG-NaF PET/CT is a feasible imaging modality that offers convenience to pts without compromising detection of metastatic disease in mGU pts. FDG alone detected fewer bone lesions than combined FDG-NaF PET/CT. Clinical trial information: NCT02496208.

293 Background: We report the safety and clinical activity of CaboNivo and CaboNivoIpi in pts with mUC and other GU tumors. Methods: Part I included 4 dose levels (DLs) (Cabo PO daily and Nivo IV q2wk): DL1 Cabo40/Nivo1, DL2 Cabo40/Nivo3, DL3 Cabo60/Nivo1, DL4 Cabo 60/Nivo3. Part II included 3 DLs (Cabo PO daily, plus NivoIpi IV q3 wk x 4 doses then Nivo q2wk): DL5 Cabo 40/Nivo1/Ipi1, DL6 Cabo40/Nivo3/Ipi1, DL7 Cabo 60/Nivo3/Ipi 1. Tumors were assessed for overall response rate (ORR) by RECIST 1.1. Adverse events (AEs) were graded (G) by NCI-CTCAE v4.0. Results: From 7/22/15-10/14/16, 40pts [mUC N=14/(plasmacytoid N=1); bladder urachal N=4; bladder squamous cell carcinoma (bSCC) N=2; germ cell tumor (GCT) N=4; castrate-resistant prostate cancer (CRPC) N=9/(neuroendocrine prostate N=1); sarcomatoid renal cell carcinoma (sRCC) N=1; trophoblastic tumor N=1); sertoli cell tumor N=1; and penile SCC N=4] were treated. Median age was 58 (range 31-77); 36 (90%) were male. AEs related to study drugs with [1] CaboNivo included G3 hyponatremia 4/24 (17%), hypophosphatemia 4/24 (17%), lipase increase 3/24 (13%), dehydration 2/24 (8%), diarrhea 2/24 (8%), fatigue 2/24 (8%), HTN 2/24 (8%), thromboembolic event 1/24 (4%), rash 1/24 (4%), chest pain 1/24 (4%), amylase increase 1/24 (4%), hyperthyroid 1/24 (4%), proteinuria 1/24 (4%), thrombocytopenia 1/24 (4%); and G4 pyelonephritis 1/24 (4%); [2] CaboNivoIpi included G3 hypophosphatemia 2/16 (13%), lipase increase 2/16 (13%), fatigue 1/16 (6%), ALT increase 1/16 (6%), and HTN 1/16 (6%). There were 2/40 (5%) G3 immune-related AEs: 1 aseptic meningitis/CaboNivo; and 1 colitis/CaboNivoIpi. There were no G5 toxicities, or DLTs. 38 pts were evaluable for response. ORR was 12/38 (32%): 1 CR (bSCC); 11 PRs (5 mUC, 1 sRCC, 1 urachal, 1 bSCC, 1 CRPC, 2 penile). SD 20/38 (53%); 9/11 responses were ongoing and 26/39 (67%) pts remain on study. Conclusions: CaboNivo and CaboNivoIpi were well tolerated with no DLTs. Responses were seen at all DLs. The recommended dose for Part I is Cabo40/Nivo3 and for Part II is Cabo40/Nivo3/Ipi1. Rare tumors such as bSCC, urachal, and penile cancer demonstrated responses. Clinical trial information: NCT02496208.

317 Background: This study investigates whether changes in 18F-FDG-PET/CT correlate with response to cabozantinib at an early time point (4 wks) versus the conventional time point of restaging (8 wks) in pts with mUC, using Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST). Methods: 68 pts with mUC in a single arm phase II clinical trial of cabozantinib underwent FDG-PET/CT scans at baseline, 4 and 8 wks. Up to 5 lesions with the highest Standard Uptake Value (SUV) were designated as target lesions. Response was determined using 2 versions of PERCIST (1 lesion vs top 5 lesions with highest FDG uptake) for FDG-PET/CT at 4 and 8 wks. PERCIST response classifications were compared to RECIST v1.1 at 8 wks. Results: 54 pts had evaluable disease. The single lesion 4 wk response by PERCIST showed 40% partial metabolic response (PMR), 33% stable metabolic disease (SMD), and 27% progressive metabolic disease (PMD). The single lesion 8 wk response by PERCIST showed 31% PMR, 31% SMD, and 38% PMD. The single lesion analysis coincided with multiple lesion PERCIST analysis in 86% of pts at 4 wks and 89% at 8 wks. The 4 wk PET/CT was predictive of the 8 wk PET/CT in 75% of single lesion and 76% of multiple lesion analyses. In lesion-based analysis, the 4 wk PET/CT was predictive of the 8 wk PET/CT in 74% of bone, 89% of lung, 77% of lymph node, 74% of soft tissue, and 58% of liver lesions. Only 42% of the 8 wk PERCIST and RECIST classifications coincided. At 8 wks, 40% showed response in FDG PET-CT restaging vs 7% complete/partial response by RECIST. Conclusions: The 4 wk PET/CT scan predicts the therapy response at the 8 wk PET/CT scan however the 4 wk scan overestimates the 8 wk response. The single lesion analysis by PERCIST correlates with the multiple lesion analysis and may have more clinical utility. In the lesion-based analysis, the 4 wk PET/CT scan is predictive of the 8 wk PET/CT for bone, lung, lymph node and soft tissue but not for liver lesions. Response classifications by PERCIST are not in agreement with response classifications by RECIST. Although the methods may be complementary, they are not interchangeable. Further studies are required to validate these findings. Clinical trial information: NCT01688999.

328 Background: NaF PET/CT is a radiotracer used for the assessment of bone metastases (mets). The goal of our study was to assess bone mets using NaF PET/CT imaging in mUC pts treated with cabozantinib. Methods: We conducted a single-arm phase II trial of cabozantinib in pts with mUC. Pts with known bone mets underwent a NaF PET/CT scan at baseline and every 8 weeks until progression. At each scan, we captured, the number of lesions per pt and per anatomical site, the maximum standardized uptake values (SUV), and serum alkaline phosphatase (AP). Bone responses (bone response (BR), stable disease (SD), progressive disease (PD)) were assigned to each pt based on the NaF report. Change from baseline to the time of best response for SUV, the number of lesions, and AP were compared between BR + SD vs PD were analyzed using an exact Wilcoxon rank sum test. The Kaplan-Meier method was used to determine the association of progression free survival (PFS) or overall survival (OS) with baseline SUV and number of lesions. Results: 68 pts enrolled on study, 22 pts had bone mets at baseline (6 bone only and 16 with bone and soft-tissue mets) (median OS 4.6 mo (95% CI: 3.4-7.7 mo), and median PFS 2.0 mo (95% CI: 1.8-3.9 mo)) of which 15 pts had follow up NaF scans. At baseline, 147 total lesions were analyzed, and the lesion distribution was: spine 29%, ribs 21%, pelvis 18%, skull 11%, femur 6%, scapula 3%, humerus 3%, sternum 3%, sacrum 3%, fibula 1%, and clavicle 0.7%. 5/15 pts had BR (and remained on study 238, 252, 182, 161, and 56 days), 4/15 SD, and 6/15 PD at best response. Sites of PD occurred in bone 10/22, bone and soft-tissue 5/22, and soft-tissue 7/22. The percent difference in number of lesions among responders and non-responders was 43% (p = 0.0039). At baseline, there was no association with PFS or OS and number of lesions (p = 0.84, p = 0.97), SUV (p = 0.83, p = 0.97), or AP (p = 0.97, p = 0.33). Conclusions: Pts with mUC and bone disease have poor overall outcome. Most bone lesions were found in the spine, ribs, and pelvis. Larger studies are needed to assess the correlation of NaF response with clinical outcome in mUC. We have an ongoing study using NaF PET to assess bone mets in mUC treated with cabozantinib/nivolumab/ipilimumab. Clinical trial information: NCT01688999.

307 Background: Translational studies have shown that shed MET levels in serum and urine of pts with UC correlate with stage and visceral metastases and that cabozantinib reverses HGF-driven UC cell growth and invasion. These data support the evaluation of cabozantinib in pts with mUC. Methods: In this phase II study, pts receive cabozantinib 60 mg daily in 28-day cycles. There are 3 study cohorts (1) mUC, (2) bone only mUC (3) metastatic rare bladder histology. The primary objective is to determine the response rate (RR) by RECIST. Response is assessed every 2 cycles. Tissue, blood, and urine were collected on all pts to test for MET/HGF and immune subsets. Results: 26 out of 55 pts have enrolled (19 M, 7 F): median age 62 (42-82) and median KPS of 80%. Primary sites include bladder (77%) and upper tract UC (23%). Prior therapy includes 30% pts with 1 regimen, 39% with 2, 15% with 3, 8% with 4 and 8% with 6. 81% of pts had visceral metastases (lung, liver and bone) and 19% lymph node only metastases. 23 pts (19 in cohort 1, 3 in cohort 2 and 1 in cohort 3) were evaluable for response after completing at least 4 weeks of therapy. In cohort 1, 2 pts achieved PR (1 remained on study for 10 months and 1 remains on study after >12 months of therapy); 7 pts had SD for at least 16 weeks (1 remained on study for 11 months); 10 had PD; 1 is too early to assess for response; 1 was removed before restaging for toxicity and 1 was removed for not meeting eligibility. The objective RR is 11% and SD 37% for a clinical benefit of 48%. In cohort 2, 1 of 3 pts had a near resolution of bone lesions on NaF PET/CT for 11 months. In cohort 3, only pt enrolled (squamous cell carcinoma of the bladder) achieved SD for 16 weeks. Mixed responses with regression of lung, bone or lymph nodes were observed in 30% of pts with PD. Grade 3/4 toxicities included: fatigue (8%), hyponatremia (8%), hypophosphatemia (8%) diarrhea (4%), thromboembolism (4%), transaminitis (4%), hypothyroidism (4%), thrombocytopenia (4%), dysphonia (4%), hypomagnesemia (4%), creatinine increase (4%) and proteinuria (4%). Conclusions: Cabozantinib has clinical activity in pts with relapsed or refractory mUC with manageable toxicities. Further studies are underway to correlate response to therapy with MET expression. Clinical trial information: NCT01688999.

e15503 Background: Assessment of tumor burden change is crucial in clinical trials and patient care. We compared cabozantinib-induced volume changes of all metastatic tumor lesions, total tumor les...

e15501 Background: CTC evaluation has the potential to guide precision cancer therapy. Ideally, this analysis can be performed on the day blood is drawn, in the same institution, using compact inst...