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Abstract
The pharmacokinetics of antibodies with varied binding kinetics were simulated to assess the role of affinity and binding microconstants (kon, koff) on tumor exposure and intra-tumoral distribution. Anti-HER2 constructs (trastuzumab, pertuzumab, VK3VH6, and conjugates with DM1 and gelonin) were produced, purified, and tested for binding and cytotoxicity in vitro, and for intra-tumoral distribution and anti-tumor efficacy in mice. Simulations demonstrated that homogeneity in intra-tumoral distribution increases with increases in koff and with decreases in kon. Interestingly, simulations also predicted that homogeneity in tumor distribution may be improved by decreasing kon and koff in parallel (without changing affinity). Relative to trastuzumab, pertuzumab exhibits similar affinity but a ~ fivefold smaller kon and koff, while VK3VH6 exhibits a similar koff but a ~ 30-fold lower kon and affinity. Conjugate concentrations associated with 50% inhibition of cell proliferation (IC50s) were found to vary with affinity, where IC50 values were similar for pertuzumab and trastuzumab, and higher for VK3VH6. Consistent with model simulations, VK3VH6 and pertuzumab demonstrated more homogeneous tumor distribution than trastuzumab. Although treatment differences were not statistically significant, pertuzumab and VK3VH6 conjugates showed trends for increased survival time relative to mice treated with trastuzumab conjugates. Our simulation and experimental results demonstrate complex relationships between antibody-antigen binding kinetics, intratumoral distribution, and efficacy. The rate constant of association, kon, is an underappreciated determinant of intra-tumoral distribution; among high-affinity antibodies, those with lower values of kon may be expected to exhibit improved intra-tumoral distribution and, potentially, efficacy.Graphical
Highlights
Immunoconjugates, including antibody–drug conjugates (ADCs), radioimmunoconjugates and immunotoxins, are a rapidly growing class of targeted cancer treatments [1–5]
Simulations were performed to investigate the intra-tumoral distribution of anti-HER2 antibodies with different binding kinetics, focusing on the percentage of HER2 receptors occupied by antibodies in layer E (Fig. 2a)
This finding relates to regions of simulation sensitivity to parameter values; for example, considering the kon and koff values of trastuzumab a starting point, intra-tumoral distribution is not predicted to be sensitive to decreases in koff (Fig. 2b), but is predicted to be sensitive to decreases in kon (Fig. 2c)
Summary
Immunoconjugates, including antibody–drug conjugates (ADCs), radioimmunoconjugates and immunotoxins, are a rapidly growing class of targeted cancer treatments [1–5]. These antibody-based therapies have proven to be successful in the treatment of hematological cancers, with somewhat less impressive results seen in the treatment of solid tumors [6, 7]. Sub-optimal efficacy in solid tumors can be explained by several factors, including limited intratumoral penetration and heterogeneous distribution These limitations have been attributed to various phenomena such as heterogeneous blood supply to the tumor, dense intratumoral stroma, increased interstitial pressure (which diminishes the pressure gradient that drives antibody transport within tumors), intra-tumoral target-mediated monoclonal antibody (mAb) elimination (acting as a “catabolic sink” for anti-cancer mAb), and heterogeneous antigen distribution [7–9]. The BSB hypothesis indicates that mAb with high affinity would bind to cells at perivascular sites, proximal to sites of antibody extravasation, limiting intratumoral distribution [8, 10]
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