Tumor Drug Penetration Measurements Could Be the Neglected Piece of the Personalized Cancer Treatment Puzzle

Abstract

Precision medicine aims to use patient genomic, epigenomic, specific drug dose, and other data to define disease patterns that may potentially lead to an improved treatment outcome. Personalized dosing regimens based on tumor drug penetration can play a critical role in this approach. State-of-the-art techniques to measure tumor drug penetration focus on systemic exposure, tissue penetration, cellular or molecular engagement, and expression of pharmacological activity. Using in silico methods, this information can be integrated to bridge the gap between the therapeutic regimen and the pharmacological link with clinical outcome. These methodologies are described, and challenges ahead are discussed. Supported by many examples, this review shows how the combination of these techniques provides enhanced patient-specific information on drug accessibility at the tumor tissue level, target binding, and downstream pharmacology. Our vision of how to apply tumor drug penetration measurements offers a roadmap for the clinical implementation of precision dosing.

Figure 1

The pathway of drug administration to the tumor response is affected by tumor drug penetration at four levels: (1) the systemic level (the concentration of the drug in the blood pool, which determines how much of the drug is available for tumor penetration), (2) the tissue level (e.g., is the drug able to distribute throughout the tumor tissue, as influenced by the tumor microenvironment), (3) the cellular or molecular engagement level (where the drug is able to engage and interact with its target at the cellular/molecular level, a proximal or direct measure of drug mechanism of action), and (4) the expression of pharmacological activity following target engagement (a distal or indirect measure of drug pharmacodynamics). All these levels will be affected by responses to treatment (bottom).

Figure 2

At clinically relevant doses, the binding of ado‐trastuzumab emtansine (T‐DM1) to human epidermal growth factor receptor 2 (HER2) expressing tumor cells is limited to the cells near functional blood vessels, and much higher doses are needed to provide a more homogeneous penetration, as shown at the microscopic level in an HER2 expressing xenograft tumor model (NCI‐N87 xenograft). (a) An immunofluorescence image of a tumor 24 hours following administration of 3.6 mg/kg of Alexa Fluor 680 tagged T‐DM1 ‐ a dose comparable to the dose used in patients ‐ to nude mice bearing NCI‐N87 flank tumors (green). Immunofluorescence staining with CD31‐AF555 (red) shows tumor vasculature, and intravenous administration and visualization of Hoechst 33342 shows functional vessels (blue) using multiplexed imaging. (b) HER2 expression (ex vivo staining with trastuzumab) in the same tumor section (white) and enlarged (c), indicating the uptake in the tumor was only sufficient to target a few cell layers. Images d, e, f show the same visualizations 24 hours following administration of 3.6 mg/kg of Alexa Fluor 680 tagged T‐DM1 and 10.8 mg/kg unlabeled trastuzumab (14.4 mg/kg total in a 1:3 ratio), indicating a more homogenous tumor penetration of T‐DM1. This dose reached many cells but did not occupy all accessible receptors in the tumor. Much higher doses up to 32 mg/kg of a combination of T‐DM1 and trastuzumab, in a 1:8 ratio (the latter to avoid antibody‐drug conjugate toxicity and improve penetration) were required in this animal model (with high HER2 expression, ~1 million receptors/cell) to reach all cells (data not shown). Red = CD31 +  staining; green = 3.6 mg/kg T‐DM1‐AlexaFluor 680 (a–c) or 3.6 mg/kg T‐DM1‐AlexaFluor 680 + 10.8 mg/kg untagged trastuzumab (d–f); white = HER2 (trastuzumab labeling of histology slide); blue = functional vessels (intravenous Hoechst 33342). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 3

Lack of correlation between human epidermal growth factor receptor 2 (HER2) assessed by immunohistochemical (IHC) and ⁸⁹Zr‐trastuzumab uptake in the same lesion of a patient in the ZEPHIR trial (NCT01565200). An HER2‐positive tumor of a patient with metastatic breast cancer with lung metastasis was visualized using (a) ¹⁸Fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT), (a marker of tumor metabolism) but not with (b) HER2 PET/CT (non‐significant tracer uptake). Pre‐treatment (tx) biopsy of a right metastasis in the middle lobe (c) shows IHC 3 +  staining (antibody recognizing the intracellular domain of the receptor). Response assessment (d) with FDG‐PET/CT shows progressive disease after three courses of ado‐trastuzumab emtansine (T‐DM1). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4

Drug development typically proceeds by optimizing molecular properties of target engagement and access (e.g., biophysical binding and cell culture methods) followed by preclinical studies (ex vivo and in vivo measurements) and eventual human trials to determine clinical endpoints. Here, we present a vision of how we can use in silico methods to help bridge the gap between these methods to a more comprehensive understanding (top). These same approaches can be used to integrate personalized data (imaging, plasma clearance, and biopsies) with computational models containing preclinical and in vitro data to develop personalized dosing schemes (bottom). MS, mass spectrometry. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 5

A vision for incorporating tumor drug penetration imaging to guide precision dosing. Noninvasive and invasive measurements can be applied to optimize treatment selection (prior to treatment initiation, left side) and to monitor and optimize drug dosing (during treatment right side. See text for further details). PK, pharmacokinetic; Tx, treatment. [Colour figure can be viewed at wileyonlinelibrary.com]