Respiratory syncytial virus (RSV) is the leading cause of
viral bronchiolitis and
pneumonia in infants and children. Currently,
palivizumab is the only approved
monoclonal antibody (mAb) for prophylaxis of RSV. However, a small percentage of patients are not protected by
palivizumab; in addition,
palivizumab does not inhibit RSV replication effectively in the upper respiratory tract. We report here the development and characterization of
motavizumab, an ultra-potent, affinity-matured, humanized mAb derived from
palivizumab. Several
palivizumab variants that enhanced the neutralization of RSV in vitro by up to 44-fold were generated; however, in vivo prophylaxis of cotton rats with these
antibodies conferred only about a twofold improvement in potency over
palivizumab. This unexpected small increase of in vivo potency was caused by poor serum pharmacokinetics and lung bio-availability that resulted from unexpectedly broad tissue binding. Subsequent analyses revealed that changes at three
amino acids arising from the affinity maturation markedly increased the non-specific binding to various tissues. Our results suggested that k(on)-driven mutations are more likely to initiate non-specific binding events than k(off)-driven mutations. Reversion of these three residues to the original sequences greatly diminished the tissue binding. The resulting mAb,
motavizumab, binds to RSV F
protein 70-fold better than
palivizumab, and exhibits about a 20-fold improvement in neutralization of RSV in vitro. In cotton rats, at equivalent concentrations,
motavizumab reduced pulmonary RSV titers to up to 100-fold lower levels than did
palivizumab and, unlike
palivizumab,
motavizumab very potently inhibited viral replication in the upper respiratory tract. This affinity-enhanced mAb is being investigated in pivotal clinical trials. Importantly, our engineering process offers precious insights into the improvement of other therapeutic mAbs.