Optimal dosing of the programmed death‑1 (PD‑1) inhibitor pembrolizumab remains incompletely defined. Historically, dose selection for PD‑1 inhibitors has been guided by measurements of peripheral receptor saturation, with the assumption that complete receptor occupancy equates to maximal pharmacodynamic effect. The study examined whether peripheral receptor occupancy reflects immune activation within systemic circulation and the tumour microenvironment in non‑small cell lung cancer.
The investigators extended a previously described physiological pharmacokinetic (PK) model that captured pembrolizumab concentrations in plasma and within tumour tissue. That PK framework was coupled with receptor occupancy parameters and with interleukin‑2 (IL‑2) induction parameters derived from ex vivo experiments reported by the authors.
Simulations were performed to compare three licensed pembrolizumab dosing regimens: 2 mg/kg every 3 weeks (Q3W), 200 mg Q3W, and 400 mg every 6 weeks (Q6W). The integrated PK–receptor occupancy–IL‑2 model was used to predict temporal changes in drug concentration, receptor engagement, and a downstream marker of T cell activation (IL‑2) in both systemic and intratumoural compartments.
Model simulations indicated that all three regimens achieved near‑complete PD‑1 receptor occupancy in both plasma and tumour compartments across the dosing intervals. This replicates the empirical observation that standard regimens yield high levels of receptor engagement.
Despite maintained receptor occupancy, predicted IL‑2 concentrations were not constant over the dosing interval. Instead, IL‑2 levels changed and tracked the pembrolizumab concentration-time profile rather than remaining tied to receptor occupancy. In other words, immune activation as proxied by IL‑2 was dynamic and correlated with circulating drug levels.
The highest simulated IL‑2 concentrations were observed with the 400 mg Q6W regimen. Changes in intratumoural IL‑2 were predicted to be less pronounced than changes in plasma IL‑2 for the regimens evaluated.
These model results suggest that achieving near‑complete peripheral PD‑1 receptor occupancy does not necessarily equate to sustained or maximal immune activation as measured by IL‑2 induction. Receptor occupancy therefore may be an inadequate standalone pharmacodynamic surrogate to guide pembrolizumab dose selection.
If immune activation follows circulating drug concentration rather than receptor saturation, dosing strategies that rely solely on receptor occupancy could mischaracterise the relationship between dose and biological effect. The findings challenge current occupancy‑based rationale for PD‑1 inhibitor dosing and underscore the necessity of identifying and validating more robust pharmacodynamic markers that better reflect the immune processes required for clinical efficacy.
The authors highlight a broader translational gap between receptor binding measurements and functional T cell responses, echoing prior ex vivo work that reported a dissociation between receptor occupancy and T cell functionality.
The article includes model‑predicted curves for a typical individual illustrating plasma and intratumoural pembrolizumab concentrations, receptor occupancy, and IL‑2 predictions. The figure demonstrates the temporal dissociation between receptor occupancy and the IL‑2 response.
All data from the study are available upon reasonable request, as stated by the authors.
The source text summarises a modelling and simulation analysis that integrates ex vivo IL‑2 induction parameters; it does not report clinical outcome data linking predicted IL‑2 changes to patient response or toxicity. Details beyond what is summarised in the abstract and article preview (such as model parameter values, population variability, and full validation metrics) are not reported in the provided source text and would require access to the full article or supplemental material.
In this translational PK–PD modelling study of pembrolizumab in non‑small cell lung cancer, receptor occupancy reached near‑complete levels across standard regimens but did not predict the modelled immune activation signal (IL‑2). The mismatch between receptor engagement and a downstream activation marker challenges receptor‑occupancy‑based dosing strategies for PD‑1 inhibitors and calls for better pharmacodynamic endpoints and clearer definitions of the immune activation necessary for optimal efficacy.