Syndax Pharmaceuticals, a Waltham, MA-based developer of treatments for cancer, has raised $6 million of a planned $7 million in a round of convertible debt financing, Joanna Horobin, the company’s CEO, confirmed. The information first appeared online in an SEC filing. The round included investments from all of the firm’s previous backers, which include San Diego venture firms Domain Associates, Avalon Ventures, and Forward Ventures as well as MPM Capital, of Boston and South San Francisco, and Pappas Ventures in Durham, NC. The funding will support the firm’s Phase II, double-blind, randomized clinical trial of the its drug, entinostat, in combination with the hormone therapy exemestane (Aromasin) for treating breast cancer, Horobin said. Syndax’s drug is intended to reprogram tumor cells to be receptive to hormone therapies.