Chicago ? A government panel gave the go-ahead Thursday for the first in a new class of anti-cancer drugs: an experimental agent that works by mobilizing the body’s natural immune system to fight prostate cancer.

If the federal Food and Drug Administration accepts the recommendation of its advisory committee, as it usually does, the drug will become the first therapeutic cancer vaccine to win regulatory approval.

Unlike vaccines that prevent diseases such as polio and measles – or, in the case of the HPV vaccine, cervical cancer – therapeutic vaccines are meant to treat an existing disease.

The new agent, known by the trade name Provenge, has been tested only in late-stage prostate cancer patients and added only a few months to their lives. But scientists hailed the drug as a step in the right direction because it could be the first of many such vaccines, also called active cellular immunotherapies.