Abstract Background Primary refractory myeloma is an important therapeutic challenge; understanding its clinical course and biology is essential if we are to recognize it early and target it therapeutically. Immunomodulatory (IMiD) drugs are widely used as induction therapy with good response rates but a small proportion of patients are refractory. The mechanism underlying IMiD resistance is poorly defined. Mutations in the cereblon pathway are a clear candidate but have not been widely looked for or reported. An important question is whether the primary refractory clone carries class-specific intrinsic resistance biology, leaving it sensitive to other non-cross reactive drugs, or if it is a universal resistance mechanism. We have examined the clinical course of patients with primary IMiD resistance, and whether they respond adequately to subsequent proteasome inhibition (PI), using the results of the largest clinical study in myeloma to date. Methods Myeloma XI is a UK based, open-label, parallel group, randomized controlled trial for newly diagnosed symptomatic myeloma patients of all ages. Treatment was with a combination of cyclophosphamide and dexamethasone plus either lenalidomide or thalidomide (CRD or CTD) for a minimum of 4 cycles (transplant eligible, TE) or 6 cycles (transplant non-eligible, TNE) or to maximum response. Those patients who had not achieved at least a minimal response or who had progressed during induction (PD) subsequently received a PI triplet (cyclophosphamide, bortezomib and dexamethasone, CVD). This abstract summarizes a preliminary analysis of these primary refractory cases, final data will be presented at the meeting. Results The study randomized 3894 patients of all ages giving adequate numbers to identify clinical/biological features in subgroups. Overall 207/3894 (5.3%) of patients had stable disease (SD) or PD at the end of the IMiD triplet. There was no significant difference between those who received thalidomide compared to lenalidomide (CTD: 110/1945, 5.7% CRD: 97/1949, 5.0%). A higher proportion of patients were refractory in the TNE pathway than TE (TE: 79/2042, 3.9%, TNE: 128/1852, 6.9%) 139 patients in the ITT population went on to receive treatment with bortezomib as part of the CVD regimen. The remainder n=69 were treated off protocol or died prior to treatment. CVD was well-tolerated in these patients with a median of 4 (1-8) cycles delivered. Of these patients 22/139 (16%) were also refractory to PI therapy whilst 57% upgraded their response compared to baseline: 32% [95%CI 24-40] to PR/MR and 25% [95%CI 18-33] to CR/VGPR. Patients with IMiD refractory disease had a significantly shorter PFS than those who responded to initial treatment median 8 vs 27 months, HR 2.10 [95% CI 1.77-2.49], p<0.001. For those who received subsequent CVD (n=139) and responded the median PFS was 19 months vs 7 months for double refractory patients. 32/50 transplant eligible patients who responded to CVD were able to go on and receive a transplant and had a PFS of 27 months. We compared the clinical and biological features of the double-refractory (n=22), IMiD refractory (n=185) and responsive (n=3349) patient groups. There was no significant difference in patient sex or median age (66 [range 41-78], 70 [38-88] and 67 [28-92] respectively). Laboratory measures suggested refractory patients had a higher burden of disease at diagnosis with a lower hemoglobin level and a higher proportion of patients with >20% plasma cells in their bone marrow biopsy. There was a higher proportion of patients with light chain only disease and the percentage of patients with ISS stage III was double-refractory 41%, IMiD refractory 45% and responsive 29%. The proportions of patients with adverse translocations and high-risk copy number abnormalities will be presented at the meeting. Conclusions We present the first detailed analysis of IMiD refractory myeloma patients at diagnosis. There was no difference in the percentage of patients refractory to the different IMiDs thalidomide and lenalidomide. Very few patients were primarily refractory to both IMiDs and proteasome inhibitors, suggesting the mechanisms of primary resistance to IMiDs and PIs do not significantly overlap. However, where this did occur outcomes were poor. The biological mechanisms behind resistance will be further informed by molecular studies of these patients' tumour samples. On behalf of the NCRI Haem Onc CSG Disclosures Pawlyn: Takeda Oncology: Consultancy; Celgene: Consultancy, Honoraria, Other: Travel Support. Davies:Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria. Kaiser:Takeda: Consultancy, Other: Travel Support; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; BMS: Consultancy, Other: Travel Support; Celgene: Consultancy, Honoraria, Research Funding; Chugai: Consultancy. Jones:Celgene: Honoraria, Research Funding. Kishore:celgene: Other: travel grant. Garg:Janssen: Other: Travel support, Research Funding, Speakers Bureau; Takeda: Other: Travel support; Novartis: Other: Travel support, Research Funding. Williams:Novartis: Honoraria; Janssen: Honoraria, Other: Travel support, Speakers Bureau; Celgene: Honoraria, Other: Travel support, Speakers Bureau; Takeda: Honoraria, Other: Travel support, Speakers Bureau; Amgen: Honoraria, Speakers Bureau. Karunanithi:Celgene: Other: Travel support, Research Funding; Janssen: Other: Travel support, Research Funding. Lindsay:BMS: Consultancy, Other: Travel support; Novartis: Other: Travel support; Takeda: Other: Travel support; Janssen: Consultancy; Celgene: Honoraria, Other: Travel support. Jenner:Celgene: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Other: Travel support; Novartis: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Other: Travel support, Research Funding; Takeda: Consultancy, Honoraria, Other: Travel support. Cook:Celgene: Consultancy, Honoraria, Research Funding, Speakers Bureau; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Sanofi: Consultancy, Honoraria, Speakers Bureau; Amgen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Takeda: Consultancy, Honoraria, Research Funding, Speakers Bureau; Glycomimetics: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria. Drayson:Abingdon Health: Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Owen:Takeda: Honoraria, Other: Travel support; Celgene: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Other: Travel support. Jackson:Takeda: Consultancy, Honoraria, Other: Travel support, Research Funding, Speakers Bureau; Janssen: Consultancy, Honoraria, Speakers Bureau; MSD: Consultancy, Honoraria, Speakers Bureau; Roche: Consultancy, Honoraria, Speakers Bureau; Celgene: Consultancy, Honoraria, Other: Travel support, Research Funding, Speakers Bureau; Amgen: Consultancy, Honoraria, Speakers Bureau. Morgan:Takeda: Consultancy, Honoraria; Bristol Meyers: Consultancy, Honoraria; Univ of AR for Medical Sciences: Employment; Janssen: Research Funding; Celgene: Consultancy, Honoraria, Research Funding.