Receptor occupation in the fjords

Abstract

AFTER several decades of development, individualized therapy is starting to take off. Herein, quantitative single cell analyses using cytometry technologies are key. An important example is the reliable measurement of receptor occupancy (RO) in therapies with biopharmaceuticals such as therapeutic monoclonal antibodies. In this issue of Cytometry Part A, two publications deal with measurement of RO. The publication by Pluim and colleagues (this issue pp. 1053–1065) reports and validates a new assay using flow cytometry. The Commentary by Huse (this issue, pp. 1046–1048) highlights a recently published assay by mass cytometry from Bringeland et al. (1). As a lucky coincidence I just met several authors of this important publication at this year’s meeting of the ESCCA (https://escca.eu/) in Bergen, Norway. This was again a high-level congress on the transit point of Cytometry applications from bench to bedside. In this fruitful environment, we decided to work together on this month’s editorial and devote it to the use of quantitative cell analysis for determining RO as an upcoming diagnostic measure in clinical diagnosis, therapy monitoring, and patient-tailored therapy. The last two decades have shown a revolution in biopharmaceutical therapeutics for various diseases. Though effective in many patients, much effort is put into discovering biomarkers providing mechanistic insight for personalized immunotherapy. In the preclinical phases, dosage and safety are extrapolated from trials in animal models. However, mice are not surrogates for the human immune response (2). The RO achieved by a given dose of a drug depends on expression levels of the target receptor, and history has shown that unknown variation of target receptor expression levels between species can have fatal consequences. In 2006, the clinical Phase I trial in anti-CD28 therapy in healthy human individuals taught the field of biopharmaceuticals a tough lesson: Unexpected low receptor levels in humans compared to monkeys dramatically increased RO, causing a cytokine storm in human subjects that was never observed in animal models (3). This could have been avoided by performing an in vitro RO assay, measuring the binding of drug relative to total receptors levels on single cells in the different species to guide dosage adjustments. Receptor levels also vary between patients and within patients over time, and changes in receptor levels can be induced by the treatment (4). The RO assay is a promising tool in dose optimization by determining the minimal anticipated biological effect level, which increases effectiveness and reduces side effects, as well as increases socioeconomical efficiency. Measuring RO on single cells with high parameter cytometry, may it be multicolor flow or mass cytometry, enables investigation of RO in complex cell populations with higher resolution (Huse K, this issue, pp. 1046–1048). Accuracy is paramount in RO measurement by flow cytometry, and a special issue of Cytometry Part B was devoted to this theme in March 2016 (5). In two above-mentioned publications— (1) and (Pluim et al. this issue, pp. 1053–1065), the authors use high-parameter cytometry for determination of RO together with multiple biomarkers in large panels of immune cell subsets in one single experiment. The authors also address pitfalls associated with these methods and demonstrate validation and standardization procedures to avoid erroneous results. Biopharmaceuticals in cancer have a great potential for success in some patients. Immune checkpoint inhibitors allow