Aileron Therapeutics, a Cambridge, Mass., developer of peptide-based drugs for cancer, raised $7 million in a second funding round. The company is at work on a new class of “stapled” peptides, or protein fragments, that it says can penetrate cell walls.

Proteins and peptides make attractive drug candidates because they can interact with other proteins in the body in ways that traditional small-molecule drugs cannot. Most proteins and peptides, however, are too large to enter cells, which limits their usefulness as drugs to interactions with cell-surface proteins. The ability to put peptides inside cells could open up entirely new avenues for drug development.

The round included Apple Tree Partners and the Novartis Venture Fund. With the new investment, Aileron has raised a total of $10.5 million, according to VentureWire (subscription required).