How to Jack up a Protein's Potency

BIOTHERAPEUTICS

Last month, scientists reported that an experimental protein strongly protects monkeys from infection by a chimeric monkey/human AIDS virus [Science, 306, 485 (2004)].

The protein, PSC-RANTES, was created by Oliver Hartley and Robin E. Offord at Centre Médical Universitaire of the University of Geneva, and coworkers by chemically modifying a protein known as RANTES. They have now described their approach in another paper that suggests that therapeutic proteins can be optimized by chemical means [Proc. Natl. Acad. Sci. USA, 101, 16460 (2004)].

PSC-RANTES inhibits HIV transmission by binding to the receptor CCR5. Transmission of HIV between people almost always takes place through CCR5, Offord says. Shutting down those receptors on cells in the monkeys' vaginas, where the drug is applied, makes the receptors unavailable to bind to the virus. The virus is unable to enter the cells.

Although PSC-RANTES is still far from being a practical drug, it is a more effective inhibitor of HIV entry than RANTES, the natural human protein that binds to CCR5.

In 1997, Offord and coworkers modified a RANTES variant to produce AOP-RANTES, which at the time was the most potent agent against HIV entry. In that molecule, the N-terminal residue of RANTES-serine-is replaced with an aminooxypentane oxime.

The hydrophobicity of the N-terminal group is key to the protein's activity. Systematic studies of AOP-RANTES led to analogs with greater hydrophobicity. When optimal hydrophobicity was achieved, subsequent optimizations focused on replacing the amino acids at the next two positions. The experiments yielded not only structure-activity data in finer detail than had ever been obtained for a protein but also PSC-RANTES, which is 50 times more potent than AOP-RANTES.

"What we did with the HIV-entry inhibitor, you can do just as well with any other biotherapeutic," Offord says. Despite two decades of use, he points out, biotherapeutics are still based on "the sequences that nature gave us." With 70-residue proteins now easily prepared by total synthesis and with hundreds of nonnatural amino acids available for structure-activity studies, it is now possible to "dissect and explore the pharmacophore of small or medium-size proteins with a finer resolution and introduce changes with a wider range of physicochemical consequences than has previously been tried," he adds.

Medicinal chemistry might also help address a key problem with therapeutic proteins: They have to be formulated as injectables. To the extent that structural modification can help proteins survive the oral route, "medicinal chemistry gives one the most control," Offord says.

Offord is a cofounder of Gryphon Pharmaceuticals, which focuses on therapeutic applications of synthetic proteins. He holds equity in the company and is a paid consultant.