Background: Arginine-depleting therapy with pegylated arginine deiminase (ADI-PEG20) was reported to have activity in advanced melanoma in early phase I-II trial, and clinical trials are currently underway in other cancers. However, the optimal patient population who benefit from this treatment is unknown.
Methods: Advanced melanoma patients with accessible tumours had biopsy performed before the start of treatment with ADI-PEG20 and at the time of progression or relapse when amenable to determine whether argininosuccinate synthetase (ASS) expression in tumour was predictive of response to ADI-PEG20.
Results: Twenty-seven of thirty-eight patients treated had melanoma tumours assessable for ASS staining before treatment. Clinical benefit rate (CBR) and longer time to progression were associated with negative expression of tumour ASS. Only 1 of 10 patients with ASS-positive tumours (ASS+) had stable disease, whereas 4 of 17 (24%) had partial response and 5 had stable disease, when ASS expression was negative (ASS-), giving CBR rates of 52.9 vs 10%, P=0.041. Two responding patients with negative ASS expression before therapy had rebiopsy after tumour progression and the ASS expression became positive. The survival of ASS- patients receiving at least four doses at 320 IU m(-2) was significantly better than the ASS+ group at 26.5 vs 8.5 months, P=0.024.
Conclusion: ADI-PEG20 is safe and the drug is only efficacious in melanoma patients whose tumour has negative ASS expression. Argininosuccinate synthetase tumour positivity is associated with drug resistance and tumour progression.