a possible breakthrough. >> this is huge news. it's a drug cocktail that seems to be as effective as chemo but without some of those awful side effects. abc's john schriffen introduces us to a woman alive because of it. >> reporter: this is a big step for 28-year-old bridget spencer. exercising, feeling better than ever, trying to put the last seven years behind her. just a week after graduating from college, she was diagnosed with stage 4 breast cancer. >> i had had 7 surgeries, 32 rounds of radiation and i had been on 14 different drugs. >> reporter: but the cancer grew through the chemo spreading to her liver, lung and bones. desperate, she turned to a drug trial. she had a more powerful cocktail called t-dm1. >> it's like a heat seeking missile with a toxic payload. travels all over the body until it meets the cancer cell and then lets off a massively toxic dose to kill off the cancer without damaging the rest of the body.