1986: Muromonab-CD3: First Therapeutic Monoclonal Antibody (1986)

Following Köhler and Milstein's 1975 hybridoma technology, it took over a decade for a monoclonal
antibody to reach clinical approval. Muromonab-CD3 (OKT3; trade name Orthoclone OKT3), a murine
IgG2a antibody targeting the CD3 epsilon chain of the T-cell receptor complex, was approved by
the FDA on 26 June 1986 for the treatment of acute allograft rejection in kidney transplant
recipients—the first therapeutic monoclonal antibody in history.

OKT3 was developed by Robert Goldstein and colleagues at Ortho Pharmaceutical using Köhler and
Milstein's hybridoma methodology. The antibody bound to the CD3 complex, initially activating
and then depleting or down-regulating T cells, thereby suppressing the immune response responsible
for acute rejection. Clinical trials demonstrated that OKT3 could reverse steroid-resistant
rejection in the majority of cases.

OKT3's clinical story illustrates both the promise and limitations of first-generation murine
antibodies: its therapeutic effect was often dramatic, but most patients developed human
anti-murine antibody (HAMA) responses that limited repeated use. These limitations drove the
engineering advances—chimerisation, humanisation, and fully human antibody technologies—that
subsequently made therapeutic antibodies broadly viable.

OKT3 was withdrawn from the market in 2010, superseded by more modern immunosuppressive agents
with better tolerability profiles. Nevertheless, its approval in 1986 opened the regulatory
pathway for all subsequent therapeutic antibodies and demonstrated that the immune system itself
could be targeted pharmacologically for therapeutic benefit.