Antibodies have proven to be effective agents in cancer imaging and therapy. One of the major challenges still facing the field is the heterogeneous distribution of these agents in tumors when administered systemically. Large regions of untargeted cells can therefore escape therapy and potentially select for more resistant cells. We present here a summary of theoretical and experimental approaches to analyze and improve antibody penetration in tumor tissue.

Pharmacokinetic analysis at the organ level provides insight into how drugs distribute throughout the body but cannot explain how drugs work at the cellular level. Here we demonstrate in vivo single cell pharmacokinetic imaging of PARP-1 inhibitors (PARPi) and model drug behavior under varying conditions. We visualize intracellular kinetics of PARPi distribution in real time, showing that PARPi reaches its cellular target compartment, the nucleus, within minutes in vivo both in cancer and normal cells in various cancer models. We also use these data to validate predictive finite element modeling. Our theoretical and experimental data indicate that tumor cells are exposed to sufficiently high PARPi concentrations in vivo and suggest that drug inefficiency is likely related to proteomic heterogeneity or insensitivity of cancer cells to DNA repair inhibition. This suggests that single cell pharmacokinetic imaging and derived modeling improves our understanding of drug action at single cell resolution in vivo.

There has been intense interest in the development of selective bioorthogonal reactions or "click" chemistry that can proceed in live animals. Until now however, most reactions still require vast surpluses of reactants because of steep temporal and spatial concentration gradients. Using computational modeling and design of pharmacokinetically optimized reactants, we have developed a predictable method for efficient in vivo click reactions. Specifically, we show that polymer modified tetrazines (PMT) are a key enabler for in vivo bioorthogonal chemistry based on the very fast and catalyst-free [4 þ 2] tetrazine/trans-cyclooctene cycloaddition. Using fluorescent PMT for cellular resolution and 18 F labeled PMT for whole animal imaging, we show that cancer cell epitopes can be easily reacted in vivo. This generic strategy should help guide the design of future chemistries and find widespread use for different in vivo bioorthogonal applications, particularly in the biomedical sciences.in vivo chemistry | pharmacokinetics | PET imaging | intravital microscopy | pretargeting T he ability to perform selective chemistries in living systems such as single cells, 3D cultures, invertebrates, or mammals would have far reaching applications in tracking biomolecules, designing new therapeutic approaches, and in visualizing medically relevant biomarkers. To date, only a few practical bioorthogonal reactions have been reported, the most popular being the Staudinger ligation and the [3 þ 2] cycloaddition "click" reaction between azides and alkynes (1, 2). The latter click reaction involves copper(I) catalyzed coupling of an azide and terminal alkyne to generate a stable triazole (2). Until recently, the necessity of the copper catalyst precluded use of this reaction in biological systems due to toxicity concerns (3, 4). Bertozzi and others elegantly solved this problem by developing several new ring strained dienophile derivatives that do not require catalysts (5-9). However, many of these derivatives have low water solubility, require complex multistep synthesis, and possess suboptimal kinetics. Our search for alternative rapid, selective, and chemically accessible coupling reactions without need for a catalyst led us and others to investigate the [4 þ 2] inverse Diels-Alder cycloaddition (10-13). We realized that this set of chemistries is more uniquely suited to biological applications and may indeed represent a universal platform technology ( Fig. 1 A and B). Specifically we and others have shown that the cycloaddition between tetrazine (Tz) and trans-cyclooctene (TCO) can proceed orders of magnitude faster than previously studied azide and alkyne click reactions and importantly does not require the action of a catalyst. This reaction has also been adapted for single cell imaging using newer fluorogenic Tz probes (14).Although initial work focused on in vitro labeling there has been a surge of recent work applying this reaction for various in vivo applications (15-17). Despite the above progress, our results with initial TCO...