Tracer kinetic modeling of a 14 kDa anti-CD8 PET tracer (2C8v144) for in vivo estimation of CD8 expression in tissue and blood, binding affinity, and total body receptor occupancy.

We are developing a 14.3 kDa anti-CD8 VHH tracer, [[Formula: see text]F]-2C8v144 (binding tracer, B), along with a nonbinding control tracer, [[Formula: see text]F]-2C8v145 (C), in order to track CD8+ (“cytotoxic”) T cells in malignant tumors during immunotherapy by Positron Emission Tomography (PET). Arterial blood concentrations of C and B were monitored in three rhesus monkeys upon i.v. injection of mass doses/kg varying approximately 5–fold for both tracers. Plasma concentrations, calculated from individual hematocrit measurements assuming no cellular uptake, were analyzed by mixed–effects compartmental modeling, by assuming the parameters of non-specific distribution and elimination (estimated based on the PET-based arterial blood concentrations) to be the same for B as for C. C exhibited linear three–compartment kinetics with a mean residence time of 26 min. Nonlinear kinetics of B suggesting saturable, reversible binding outside of circulating blood were described by an average association rate constant, [Formula: see text], and an equilibrium dissociation constant, [Formula: see text]. Binding in blood to circulating CD8+ cells was described by a different association rate constant, [Formula: see text], and the same [Formula: see text]. The total body content of CD8 receptors was estimated at 1.7 nmol/kg body weight, with 2.7% of the total present in blood. The estimate of [Formula: see text], 0.0097/(min·nM), was > 50–fold lower than [Formula: see text] determined in vitro by Surface Plasmon Resonance (SPR, 0.50/(min·nM), yet the estimated [Formula: see text], 0.23 nM, was similar to the SPR estimate (0.13 nM) suggesting that the model informs about affinity. The model also yields predicted total body receptor occupancies and plasma concentrations of unbound B, i.e. the arterial input function needed for analyzing tracer kinetics in malignant tumors.