Red blood cells (RBC) have great potential as drug delivery systems, capable of producing unprecedented changes in pharmacokinetics, pharmacodynamics, and immunogenicity. Despite this great potential and nearly 50 years of research, it is only recently that RBC-mediated drug delivery has begun to move out of the academic lab and into industrial drug development. RBC loading with drugs can be performed in several ways-either via encapsulation within the RBC or surface coupling, and either ex vivo or in vivo-depending on the intended application. In this review, we briefly summarize currently used technologies for RBC loading/coupling with an eye on how pharmacokinetics is impacted. Additionally, we provide a detailed description of key ADME (absorption, distribution, metabolism, elimination) changes that would be expected for RBC-associated drugs and address unique features of RBC pharmacokinetics. As thorough understanding of pharmacokinetics is critical in successful translation to the clinic, we expect that this review will provide a jumping off point for further investigations into this area.Pharmaceutics 2020, 12, 440 2 of 21 of the research enterprise encompassing the design of drug delivery systems (DDS)-liposomes, antibody-drug conjugates, and polymeric nanocarriers, to name a few.Nevertheless, approaches to use RBCs as carriers for pharmacological agents have recently gained significant and rapidly growing attention. Progress in this field is rapidly diversifying and accelerating towards potentially clinically useful products. Many groups are now investigating the use of RBCs in drug delivery and are making significant contributions, leading to breakthrough findings and upbeat investments. Several RBC-based drug delivery approaches have entered clinical trials, including RBC-encapsulated asparaginase (Erytech, Phase 3) and dexamethasone (EryDel, Phase 3). Novel advanced strategies are emerging, including genetic molecular modifications of RBC [2,3], modulation of the immune system by RBC-coupled antigens [3,4], and vascular transfer of RBC-coupled nanocarriers (RBC hitchhiking) [5][6][7].Both encapsulation into and coupling to the surface of RBC fundamentally transform the key parameters of absorption, distribution, metabolism, and elimination (ADME) of drugs and drug delivery systems (DDS), including diverse nanocarriers. To our knowledge, studies of the pharmacokinetics (PK) and pharmacodynamics (PD) of RBC-based DDS are lacking, despite great relevance for industrial development and clinical utility.In order to help to close this gap of knowledge, in this paper we undertook the first attempt to define specific, salient parameters controlling behavior of RBC/DDS in the body. Our goal is to provide the modular framework for experimental and theoretical pre-clinical and clinical investigations of ADME-PK-PD features of RBC-based drug delivery.

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Extensive studies on focused ultrasound (FUS)-mediated drug delivery through the blood–brain barrier have been published, yet little work has been published on FUS-mediated drug delivery through the blood-spinal cord barrier (BSCB). This work aims to quantify the delivery of the monoclonal antibody trastuzumab to rat spinal cord tissue and characterize its distribution within a model of leptomeningeal metastases. 10 healthy Sprague–Dawley rats were treated with FUS + trastuzumab and sacrificed at 2-h or 24-h post-FUS. A human IgG ELISA (Abcam) was used to measure trastuzumab concentration and a 12 ± fivefold increase was seen in treated tissue over control tissue at 2 h versus no increase at 24 h. Three athymic nude rats were inoculated with MDA-MB-231-H2N HER2 + breast cancer cells between the meninges in the thoracic region of the spinal cord and treated with FUS + trastuzumab. Immunohistochemistry was performed to visualize trastuzumab delivery, and semi-quantitative analysis revealed similar or more intense staining in tumor tissue compared to healthy tissue suggesting a comparable or greater concentration of trastuzumab was achieved. FUS can increase the permeability of the BSCB, improving drug delivery to specifically targeted regions of healthy and pathologic tissue in the spinal cord. The achieved concentrations within the healthy tissue are comparable to those reported in the brain.