CAMBRIDGE, Mass. ( TheStreet) -- Vertex Pharmaceuticals ( VRTX) has slipped from the radar screen of biotech investors, many of whom are still recovering from the American Society of Clinical Oncology annual meeting. But Vertex is presenting additional data on its combination therapy for cystic fibrosis at the European Cystic Fibrosis Society (ECFS) conference on June 13. Investors should take notice. The first look at positive results from the combination of VX-661 and Kalydeco released by Vertex in April spiked shares from $52 to $85, as it indicated the Vertex drugs could effectively address a much larger share of the cystic fibrosis patient population. Kalydeco is currently only approved for the 4 percent of cystic fibrosis patients carrying the G551D mutation in the CFTR gene. Clearly, expanding the size of the addressable market would be quite meaningful to Vertex. Cystic fibrosis is caused by dysfunction in the CFTR gene, which severely limits the ability of the CFTR protein to express in the epithelial cells of the lungs, intestines, and exocrine glands. Malfunctioning CFTR proteins inhibits the ability of chloride ions to cross those membranes, which ultimately creates a thick mucus that impedes the proper functioning of the organ and leads to numerous opportunistic infections. In general, a drug (or combinations) needs to address three issues to be effective as a cystic fibrosis therapy: Production of a functional CFTR protein, trafficking the protein to the proper place on the cell membrane, and gating it through the cell membrane. Kalydeco is effective in the G551D population because these patients have functional CFTR proteins that are properly trafficked but have issues with gating. Kalydeco works as a "potentiator" by essentially opening the gates and letting chloride ions pass through. Kalydeco is not effective in the other 96 percent of cystic fibrosis patients because they have problems with either the production of the protein and/or trafficking it to the proper spot. This is where combination therapy comes into play. A "corrector" drug could handle protein function and trafficking while Kalydeco takes care of gating. Vertex has three "correctors" in development (VX-809, VX-661, and VX-983) which attempt to address issues in production and trafficking. The ultimate goal is to develop a successful corrector/Kalydeco combination that gets enough functional protein across the cell membrane to produce a benefit for cystic fibrosis patients similar to what's been demonstrated by Kalydeco monotherapy in the G551D mutation population. Vertex's early combination data improved lung function only about half as well as Kalydeco. Still, the improvement in lung function and decrease in sweat chloride production were statistically significant, providing a clear proof of concept. It looks more like a matter of when, not if, Vertex will be able to develop an effective combination treatment for the much larger CF patient populations.