 ROCKVILLE, Md., April 14, 2021 PRNewswire -- Synthetic Biologics, Inc. NYSE American: SYN, a diversified clinical-stage company leveraging the microbiome to develop therapeutics designed to prevent and treat gastrointestinal GI diseases in areas of high unmet need, today announced enrollment has commenced and the first patient has been dosed in its Phase 1b2a clinical trial of SYN-004 ribaxamase in allogeneic hematopoietic cell transplant HCT recipients for the prevention of acute graft-versus-host-disease aGVHD. 