Fern Saitowitz’s advanced breast cancer was controlled for about a year by the drug Herceptin and a toxic chemotherapy agent. But her hair fell out, her fingernails turned black and she was constantly fatigued.

She switched to an experimental treatment, which also consisted of Herceptin and a chemotherapy agent. Only this time, the two drugs were attached to each other, keeping the toxic agent inactive until the Herceptin carried it to the tumor. Side effects, other than temporary nausea and some muscle cramps, vanished.

“I’m able to live a normal life,” said Ms. Saitowitz, 47, a mother of two young children in Los Angeles. “I haven’t lost any of my hair.”

The experimental treatment, called T-DM1, is a harbinger of a new class of cancer drugs that may be more effective and less toxic than many existing treatments. By harnessing antibodies to deliver toxic payloads to cancer cells, while largely sparing healthy cells, the drugs are a step toward the “magic bullets” against cancer first envisioned by Paul Ehrlich, a German Nobel laureate, about 100 years ago.