Abstract Implementing PK-guided dosing with monoclonal antibodies (mAbs) in oncology has been for long impaired by difficulties in developing appropriate (i.e., time- and cost-effective) bioanalytical methods suitable for routine Therapeutic Drug Monitoring, plus concerns regarding the exact PK/PD relationships of several biologics possibly blurred by the TMDD phenomenon. To help lifting these issues, we have developed and cross-validated a multiplex LC-MS/MS method allowing to assay simultaneously up to 8 mAbs in plasma, including several immune checkpoint inhibitors (i.e., atezolizumab, bevacizumab, cetuximab, ipilimumab, nivolumab, pembrolizumab, rituximab, trastuzumab). Using a ready-to-use kit (mAbXmise), the method proved to be simple and rapid - covering a concentration range of 2-100 µg/ml, in line with plasma concentrations usually expected with mAbs. Inter and intra-assay precision were both <15% and accuracy was comprised between 90.1 and 111.1%, thus meeting the requirements of current EMA guidelines for validating bioanalytical methods. Cross-validation using reference LC-MS/MS or ELISA methods was performed on 7 mAbs (excluding atezolizumab since no reference method was available) with a satisfactory mean absolute bias of 10.6% (3.0-19.9%). This LC-MS/MS method was next used as part of the Cetuximax trial (NCT-04218136), an open, non-randomized, single arm, multicentric study aiming at determining the PK/PD relationships of Cetuximab in Head-and-Neck cancer patients with monitoring of both Cmax and Cmin levels. Patients are all treated with the 250 mg/m² QW schedule. Previous works have suggested that Cetuximab trough levels < 34 µg/ml were associated with higher risk for treatment failure in Head-and-Neck patients. Patients were repeatedly sampled in a longitudinal fashion at their Cmax plus trough levels and individual PK parameters were derived using a pop-PK approach. Preliminary results on 90 samples collected from the first 25 out of the 110 patients to be included have confirmed the marked inter-individual variability in Cetuximab exposure (i.e., >56% on trough levels and >55% on Cmax values). Mean Cetuximab trough level was 54 ± 30 µg/ml (range: 12-104 µg/ml) and mean Cmax value was 211 ± 116 µg/ml (range: 75-418 µg/ml), with several individuals (8 out of 25, i.e., 32%) showing plasma exposure below the expected trough levels associated with efficacy. Although preliminary, our data confirm that PK variability is massive with mAbs. This observation suggests that Therapeutic Drug Monitoring, using appropriate LC-MS/MS method, could help appraising interpatient variability at bedside and detecting individuals with exposure levels out of the range usually observed. Citation Format: Clemence Marin, Nihel Khoudour, Aurélien Millet, Dorothée Lebert, Pauline Bros, Fabienne Thomas, David Ternant, Jerôme Guitton, Mourad Hamimed, Benoit Blanchet, Sebastien Salas, Joseph Ciccolini. A multiplex LC-MS/MS method for assaying mAbs in oncology: Application to the CETUXIMAX GPCO-Unicancer trial on cetuximab pharmacokinetics in head and neck cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1144.