A bipartisan group of senators asked the head of a pharmaceutical company why the cost of a 40-year-old, cancer-fighting drug has spiked 1,400 percent over the last four years. A letter released Monday by Sen. Catherine Cortez Masto, D-Nev., Sen. Susan Collins, R-Maine, and Sen. Claire McCaskill, D-Mo., is the first step toward a Senate probe into Tri-Source Pharma and its subsidiary NextSource Biotechnology.