Rhythm, a Boston biotechnology company developing peptide therapeutics to treat such metabolic diseases as obesity and diabetes, said it has raised $8 million from Pfizer Venture Intestments, the venture capital arm of the drug company Pfizer and a new investor in Rhythm.

That additional $8 million completes a $33 million Series B financing. The round had participation from existing investors MPM Capital, New Enterprise Associates, Third Rock Ventures, and Ipsen, the company said.