SS1P is an immunotoxin comprised of the variable fragment of a mesothelin-targeted antibody linked to a bacterial toxin. Although SS1P induces apoptosis in mesothelioma cell lines, which express mesothelin, only minor antitumour activity was observed in Phase I trials in patients with mesothelioma, primarily because patients developed neutralizing antibodies to SS1P. To circumvent this, Hassan et al. treated patients with pentostatin and cyclophosphamide to deplete B cells and T cells prior to SS1P treatment. This regimen resulted in major tumour regression in three of ten patients with chemotherapy-refractory mesothelioma.