PET a better guide for anti-PD-1 outcomes and discontinuation

By Michael Woodhead

29 Aug 2018

PET-CT scans can better predict long-term benefit of checkpoint inhibitor treatment than CT and also help guide discontinuation of therapy, an Australian study in patients with malignant melanoma has shown.

The increasing use of anti-PD-1 based immune checkpoint inhibitor therapy has resulted in good clinical outcomes for patients with many kinds of cancers including those who appear to show only partial response (PR) or stable disease (SD) based on CT imaging criteria, according to researchers at the University of Sydney.

Writing in Annals of Oncology, they say patients with incomplete responses to checkpoint inhibitors on CT may not have residual disease and they hypothesised that additional functional information provided by PET imaging (complete metabolic response) would help identify patients with PR or SD who may be free of viable tumours.

In a retrospective study of 104 metastatic melanoma patients treated with anti-PD-1 based immunotherapy, they compared the predictive ability of 18F-FDG PET and CT imaging at baseline with one year responses to therapy.

Their analysis showed that with CT imaging, 28% of patients had CR at one year, 66% had PR and 6% had SD. With PET scans, 75% of patients had CMR at one year, 16% partial metabolic response (PMR) and 9% stable or progressive metabolic disease (SMD/PMD) on PET.

The researchers remarked that a high proportion of the patients with PR on CT (68%) had a CMR with PET. They also noted that patients with CMR had excellent medium-term progression free survival (PFS), similar to those with CR on CT.

Of the 78 patients with CMR at one year, the median time on treatment was 20.7 months, 78% had discontinued treatment with median follow-up post discontinuation of 14.5 months, and 75 (96%) remained progression-free.

In contrast, there was a significant difference in PFS in patients with CMR on PET compared with non-CMR, with 100% and 57% progression-free at one year post imaging, respectively.

“Whilst only a small proportion of patients have a CR at one year, most patients with a PR have CMR on PET. Almost all patients with CMR at 1-year have ongoing response to therapy thereafter. PET may have utility in predicting long-term benefit and help guide discontinuation of therapy,” they concluded.

The researchers said PET had already been shown to be useful in patients with lymphoma for assessing response to treatment with checkpoint inhibitors and predicting prognosis. The new findings suggested it could also have a role in other cancers.

“Immunotherapy can induce long-term responses in a subgroup of patients across most cancer types, and whether 18F-FDG PET may play an important role in response evaluation should be explored,” they wrote.

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