Addressing drug effects on cut point determination for an anti-drug antibody assay

The effect of trough levels of a monoclonal antibody drug (drugB) on screening cut point (CP) determination for an anti-drug antibody (ADA) assay was scrutinized and the conclusions substantiated by data from a phase 3 cancer clinical study. The ADA assay utilized an acid dissociation step and either 0 or 100 μg/ml drugB was added to the samples prior to obtaining the signals used for CP calculations. Serum samples from three different drug-naive populations were tested (healthy individuals, cancer patients enrolled in the drugB clinical trial and cancer patients whose serum samples were available commercially). For the same disease state samples, both the screening CP and confirmation CP were different when calculated during validation or from study sample analysis. It is reasonable to assume that variability was due to the patient heterogeneity, as they could have been at distinct stages of disease progression, and/or taking different medications, amongst other differences. The patients enrolled in the clinical trial were stratified as per protocol and hence represented a more homogeneous population. Drug effects on CP may be population dependent and also assay dependent.