GSK, the FTSE 100 drugmaker, said its immunotherapy Jemperli (dostarlimab) produced a complete response in 76% of patients with a genetically defined form of locally advanced rectal cancer.
The interim results come from AZUR-1, a phase II trial of dostarlimab used alone in adults with stage II and III disease that is microsatellite instability-high or mismatch repair deficient, meaning the tumours cannot properly repair DNA damage.
In an exploratory analysis, 110 of 144 evaluable patients had no evidence of cancer after treatment, confirmed by endoscopy, radiology or tissue analysis.
Of these, 98 (68%) had a clinical complete response sustained for at least 12 months without surgery, as assessed by investigators, and 12 more were confirmed through tissue analysis after surgery.
No recurrences have been reported among responders after a median follow-up of more than two years.
The data supported the recent US approval of Jemperli for this indication and will be presented in a late-breaking oral session at the European Society for Medical Oncology congress.
The trial's primary endpoint, a 12-month clinical complete response judged by independent central review, is still maturing. So far 47% of 116 evaluable patients (54) have reached it, with 38 more under observation.
Standard treatment currently involves chemotherapy, radiation and surgery, which can cause lasting bowel, urinary and sexual dysfunction, secondary cancers and infertility.
Serious adverse reactions occurred in 20% of patients, and 3% stopped treatment because of side effects. The most common were fatigue (32%), rash (22%), diarrhoea (18%) and itching (17%).
Only 5% to 10% of rectal cancers have this genetic characteristic.
Hesham Abdullah, GSK's global head of oncology R&D, said avoiding chemotherapy, radiation and surgery "may now be possible" for some patients.
Regulatory reviews are continuing in other countries.
