A phase 1b clinical trial optimizing regulatory T cell depletion in combination with platinum-based chemotherapy in thoracic cancers

<b>Background</b>: Single-agent cyclophosphamide can deplete regulatory T-cells (Treg). We aimed to determine optimal dosing and scheduling of oral cyclophosphamide, alongside pemetrexed-based chemotherapy, to deplete Treg in mesothelioma or non-small-cell lung cancer patients. <b>Methods</b>: 31 Patients received pemetrexed ± cisplatin or carboplatin on day 1 of a 21-day cycle (maximum 6 cycles). From cycle two, patients received cyclophosphamide, 50 mg/day, with intrapatient escalation to maximum 100/150 mg/day alternately. Immunological changes were examined by flow cytometry. Primary endpoint was Treg proportion of CD4⁺ T-cells, with doses tailored to target Treg nadir &lt;4%. <b>Results</b>: Reduction in Treg proportion was observed on day 8 of all cycles, and was not augmented by cyclophosphamide. Few patients achieved the &lt;4% Treg target. Treg proliferation reached nadir one week after chemotherapy, and peaked on day 1 of the subsequent cycle. Efficacy parameters were similar to chemotherapy alone. Seventeen percent of patients ceased cyclophosphamide due to toxicity. <b>Conclusions</b>: Specific Treg depletion to the degree seen with single-agent cyclophosphamide was not observed during pemetrexed-based chemotherapy. This study highlights the poor evidence basis for use of cyclophosphamide as an immunotherapeutic in combination with chemotherapy, and the importance of detailed flow cytometry studies. <b>Trial registration</b>: Clinical trial registration: www.anzctr.org.au identifier is ACTRN12609000260224.