This is a double-blind, comparative, randomized phase III clinical study (CS) of the efficacy and safety of monotherapy with BCD-264 and Darzalex in subjects with relapsed and refractory multiple myeloma previously treated with proteasome inhibitors and immunomodulatory drugs, and who had disease progression on prior therapy. Subjects will receive the study therapy for a total of 26 cycles (about 104 weeks from randomization) or until disease progression or signs of unacceptable toxicity, whichever occurs first. The blinded treatment period (up to 6 therapy cycles) will last from randomization to the open label administration of BCD-264 on Day 1 of Cycle 7. The open-label therapy period will start with the open-label administration of BCD-264 on Day 1 of Cycle 7 and end with the EOT visit (maximum 20 therapy cycles). The main objective of the study is to confirm the comparability of the efficacy and safety profiles of BCD-264 and Darzalex as monotherapy for relapsed and refractory multiple myeloma in subjects previously treated with proteasome inhibitors and immunomodulatory drugs, and who had disease progression on prior therapy, which is consistent with the generally accepted practice and recommendations of Russian and international regulatory documents for the development of a biosimilar medicinal product.