Functional nanoparticles incorporating active-targeting systems, such as those targeting folic acid receptor and αVβ3 integrin, are tumor-targeted drug delivery systems (DDS) designed to achieve tumor-selective pharmacological effects [1,2]. However, the discrepancy in anticancer efficiency of these systems between animal tumor models and clinical trials is often problematic, despite good preclinical pharmacokinetic and pharmacodynamic data of anticancer activity. Zamboni et al. evaluated the differences in cisplatin exposure of tumor stroma after administering PEGylated liposome-encapsulated or free cisplatin using microdialysis systems. They revealed, for the first time, that the lack of expected treatment efficiency for PEGylated liposome formulations with good tumor retention was due to lesser drug release from liposomes into the tumor stroma [3]. Thus, quantification of total drug accumulated in a tumor tissue without distinguishing between the amount of drug that remains encapsulated in nanoparticles and that which has been released, as in conventional pharmacokinetic analysis, may lead to an overestimation of the tissue distribution potential of the DDS. Inferring from the study of Zamboni et al. [3], discrepancy between the results of preclinical and clinical trials may be because of the fact that pharmacokinetics in tumors cannot be evaluated based on the net amount of drug released from nanoparticles in the tumor and is rather “black-boxed”.

Imaging is the most important method for qualitative evaluation of three-dimensional distribution of nanoparticles and for distinguishing between nanoparticle distribution in the intracellular and extracellular matrix. In a previous study, intratumoral pharmacokinetics of nanoparticles was evaluated in vivo and in situ using real-time two-photon excitation fluorescence imaging and time-lapse imaging [4]. Other approaches involving a transparency treatment of tissues to evaluate the three-dimensional distribution of drugs have been reported [5]. These studies have contributed to an intuitive understanding of real-time local distribution, metabolism, and elimination of drugs.

However, labeled substances used for imaging are generally attached to nanoparticles, and the image data only provide pharmacokinetic information about the nanoparticles and not about the small molecules obviating the possibility of determining their release from the nanoparticles. Therefore, imaging methods are not suitable for pharmacokinetic analysis of nanoparticle-encapsulated drugs.

To overcome the problem associated with imaging, we focused on a microdialysis method [3,6]. Microdialysis, which is a classical recovery method for neurotransmitters in the brain, allows a perfusate to flow through a dialysis membrane embedded in the tissue; the perfusate is collected at regular time intervals to evaluate changes in the concentrations of small molecules. A drug released from nanoparticles using a functional drug release system can be quantified over time by applying microdialysis to the tumor tissue. If the quantified intratumoral drug concentration–time profiles can be formulated using compartmental analysis, evaluation of the net tumor transfer rate constants and tumor excretion rate constants may be possible. These rate constants may represent the intrinsic functionality of DDS nanoparticles. We hypothesized that compartmental analysis, reflecting intratumoral concentrations, may provide clues to elucidate the discrepancy between anticancer efficiency of a DDS in animal tumor models and clinical trials.

The aim of this study was to evaluate the targeting and sustained release of doxorubicin (DOX) encapsulated via amide linkages in polymeric micelles in Walker 256 tumor-bearing model rats by constructing a PK-PD model that allows the intratumoral pharmacokinetic analysis. It is expected to take time for DOX to be released from the polymeric micelles distributed in the tumor. Hence, the compartmental model, which assumes that the drug is uniformly distributed throughout the body immediately after administration, is expected to have limitations as a PK model. To address this problem, we described the process of intramicellar DOX release into the tumor using a distribution delay function. In this article, we discuss the informatics of our approach as a methodology for studying intratumoral kinetics.