(Recasts to include bladder cancer trial failure, adds analyst comments, paragraphs 6-7 and 12, share price paragraph 8)

May 31 (Reuters) - Gilead Sciences' Trodelvy failed to improve survival for patients with advanced bladder cancer and only modestly extended the lives of previously treated patients with late-stage lung cancer in a pair of clinical trials, raising questions about growth prospects for the medicine.

Trodelvy has accelerated U.S. approval for treating advanced urothelial cancer, but Gilead on Thursday said a large trial failed to confirm that the drug improved survival.

The study also linked Trodelvy to a higher number of deaths compared to chemotherapy due to side effects including infection.

On Friday, trial data presented at the American Society of Clinical Oncology meeting in Chicago showed that Trodelvy improved survival by just 1.3 months more than chemotherapy for patients with advanced lung cancer, a difference that was not statistically significant.

The company in January said that the non-small cell lung cancer (NSCLC) trial had failed to meet its main goal.

"We believe that Gilead's Trodelvy will likely lose its FDA-labeled indication for metastatic urothelial carcinoma, or bladder cancer," Leerink analyst Daina Graybosch said in a research note on Friday.

The bladder indication is estimated to account for about 10% of Trodelvy sales, which totaled just over $1 billion in 2023.

Gilead's shares were down 1% at $63.20 on Nasdaq.

Trodelvy, an antibody-drug conjugate, is currently approved in the U.S. for patients with two specific types of advanced breast cancer and bladder cancer.

Gilead said the 603-patient lung cancer trial showed a survival improvement of 3.5 months for patients given Trodelvy whose tumors had not responded to a last round of immunotherapy.

For patients whose lung cancer had responded to their last immunotherapy, overall survival was a month longer for the chemotherapy group.

"We remain cautious on the potential for Gilead to get Trodelvy on the market in lung cancer based on this study," RBC Capital Markets analyst Brian Abrahams said in a note on Friday.

Serious side effects were reported by 67% of Trodelvy patients and 76% of chemotherapy patients. The most common side effects for Trodelvy were fatigue, diarrhea and hair loss.

Gilead is also studying Trodelvy in combination with Merck's immunotherapy Keytruda as an initial treatment for patients with NSCLC, the most common lung cancer.

Results for a small subset of patients in one of those ongoing studies has shown they lived a median of 13.1 months before their cancer worsened. That is an improvement over the seven- to eight-month progression-free survival seen in Keytruda trials, Bilal Piperdi, Gilead's vice president of clinical oncology, said. (Reporting By Deena Beasley Editing by Bill Berkrot)