Monoclonal antibody-based therapies in cancer: advances and challenges

Anticancer Drug Delivery: An Update on Clinically Applied Nanotherapeutics

The development of chemotherapy using conventional anticancer drugs has been hindered due to several drawbacks related to their poor water solubility and poor pharmacokinetics, leading to severe adverse side effects and multidrug resistance in patients. Nanocarriers were developed to palliate these problems by improving drug delivery, opening the era of nanomedicine in oncology. Liposomes have been by far the most used nanovectors for drug delivery, with liposomal doxorubicin receiving US FDA approval as early as 1995. Antibody drug conjugates and promising drug delivery systems based on a natural polymer, such as albumin, or a synthetic polymer, are currently undergoing advanced clinical trials or have received approval for clinical applications. However, despite attractive results being obtained in preclinical studies, many well-designed nanodrugs fell short of expectations when tested in patients, evidencing the gap between nanoparticle design and their clinical translation. The aim of this review is to evaluate the extent of nanotherapeutics used in oncology by providing an insight into the most successful concepts. The reasons that prevent nanodrugs from expanding to clinic are discussed, and the efforts that must be taken to take full advantage of the great potential of nanomedicine are highlighted.

Bystander killing effect of DS-8201a, a novel anti-human epidermal growth factor receptor 2 antibody-drug conjugate, in tumors with human epidermal growth factor receptor 2 heterogeneity

Antibody–drug conjugates deliver anticancer agents selectively and efficiently to tumor tissue and have significant antitumor efficacy with a wide therapeutic window. DS‐8201a is a human epidermal growth factor receptor 2 (HER2)‐targeting antibody–drug conjugate prepared using a novel linker‐payload system with a potent topoisomerase I inhibitor, exatecan derivative (DX‐8951 derivative, DXd). It was effective against trastuzumab emtansine (T‐DM1)‐insensitive patient‐derived xenograft models with both high and low HER2 expression. In this study, the bystander killing effect of DS‐8201a was evaluated and compared with that of T‐DM1. We confirmed that the payload of DS‐8201a, DXd (1), was highly membrane‐permeable whereas that of T‐DM1, Lys‐SMCC‐DM1, had a low level of permeability. Under a coculture condition of HER2‐positive KPL‐4 cells and negative MDA‐MB‐468 cells in vitro, DS‐8201a killed both cells, whereas T‐DM1 and an antibody–drug conjugate with a low permeable payload, anti‐HER2‐DXd (2), did not. In vivo evaluation was carried out using mice inoculated with a mixture of HER2‐positive NCI‐N87 cells and HER2‐negative MDA‐MB‐468‐Luc cells by using an in vivo imaging system. In vivo, DS‐8201a reduced the luciferase signal of the mice, indicating suppression of the MDA‐MB‐468‐Luc population; however, T‐DM1 and anti‐HER2‐DXd (2) did not. Furthermore, it was confirmed that DS‐8201a was not effective against MDA‐MB‐468‐Luc tumors inoculated at the opposite side of the NCI‐N87 tumor, suggesting that the bystander killing effect of DS‐8201a is observed only in cells neighboring HER2‐positive cells, indicating low concern in terms of systemic toxicity. These results indicated that DS‐8201a has a potent bystander effect due to a highly membrane‐permeable payload and is beneficial in treating tumors with HER2 heterogeneity that are unresponsive to T‐DM1.

Design, synthesis, and evaluation of cisplatin-containing EGFR targeting bioconjugates as potential therapeutic agents for brain tumors

The aim of this study was to evaluate four different platinated bioconjugates containing a cisplatin (cis-diamminedichloroplatinum [cis-DDP]) fragment and epidermal growth factor receptor (EGFR)-targeting moieties as potential therapeutic agents for the treatment of brain tumors using a human EGFR-expressing transfectant of the F98 rat glioma (F98_EGFR) to assess their efficacy. The first two bioconjugates employed the monoclonal antibody cetuximab (C225 or Erbitux^®) as the targeting moiety, and the second two used genetically engineered EGF peptides. C225-G₅-Pt was produced by reacting cis-DDP with a fifth-generation polyamidoamine dendrimer (G₅) and then linking it to C225 by means of two heterobifunctional reagents. The second bioconjugate (C225-PG-Pt) employed the same methodology except that polyglutamic acid was used as the carrier. The third and fourth bioconjugates used two different EGF peptides, PEP382 and PEP455, with direct coordination to the Pt center of the cis-DDP fragment. In vivo studies with C225-G₅-Pt failed to demonstrate therapeutic activity following intracerebral (ic) convection-enhanced delivery (CED) to F98_EGFR glioma-bearing rats. The second bioconjugate, C225-PG-Pt, failed to show in vitro cytotoxicity. Furthermore, because of its high molecular weight, we decided that lower molecular weight peptides might provide better targeting and microdistribution within the tumor. Both PEP382-Pt and PEP455-Pt bioconjugates were cytotoxic in vitro and, based on this, a pilot study was initiated using PEP455-Pt. The end point for this study was tumor size at 6 weeks following tumor cell implantation and 4 weeks following ic CED of PEP455-Pt to F98 glioma-bearing rats. Neuropathologic examination revealed that five of seven rats were either tumor-free or only had microscopic tumors at 42 days following tumor implantation compared to a mean survival time of 20.5 and 26.3 days for untreated controls. In conclusion, we have succeeded in reformatting the toxicity profile of cis-DDP and demonstrated the therapeutic efficacy of the PEP455-Pt bioconjugate in F98 glioma-bearing rats.

Antibody Production in Plants and Green Algae

Monoclonal antibodies (mAbs) have a wide range of modern applications, including research, diagnostic, therapeutic, and industrial uses. Market demand for mAbs is high and continues to grow. Although mammalian systems, which currently dominate the biomanufacturing industry, produce effective and safe recombinant mAbs, they have a limited manufacturing capacity and high costs. Bacteria, yeast, and insect cell systems are highly scalable and cost effective but vary in their ability to produce appropriate posttranslationally modified mAbs. Plants and green algae are emerging as promising production platforms because of their time and cost efficiencies, scalability, lack of mammalian pathogens, and eukaryotic posttranslational protein modification machinery. So far, plant- and algae-derived mAbs have been produced predominantly as candidate therapeutics for infectious diseases and cancer. These candidates have been extensively evaluated in animal models, and some have shown efficacy in clinical trials. Here, we review ongoing efforts to advance the production of mAbs in plants and algae.

Structural characterization of antibody drug conjugate by a combination of intact, middle-up and bottom-up techniques using sheathless capillary electrophoresis - Tandem mass spectrometry as nanoESI infusion platform and separation method

Antibody-drug conjugates (ADCs) represent a fast growing class of biotherapeutic products. Their production leads to a distribution of species exhibiting different number of conjugated drugs overlaying the inherent complexity resulting from the monoclonal antibody format, such as glycoforms. ADCs require an additional level of characterization compared to first generation of biotherapeutics obtained through multiple analytical techniques for complete structure assessment. We report the development of complementary approaches implementing sheathless capillary electrophoresis-mass spectrometry (sheathless CE-MS) to characterize the different aspects defining the structure of brentuximab vedotin. Native MS using sheathless CE-MS instrument as a nanoESI infusion platform enabled accurate mass measurements and estimation of the average drug to antibody ratio alongside to drug load distribution. Middle-up analysis performed after limited IdeS proteolysis allowed to study independently the light chain, Fab and F(ab')2 subunits incorporating 1, 0 to 4 and 0 to 8 payloads respectively. Finally, a CZE-ESI-MS/MS methodology was developed in order to be compatible with hydrophobic drug composing ADCs. From a single injection, complete sequence coverage could be achieved. Using the same dataset, glycosylation and drug-loaded peptides could be simultaneously identified revealing robust information regarding their respective localization and abundance. Drug-loaded peptide fragmentation mass spectra study demonstrated drug specific fragments reinforcing identification confidence, undescribed so far. Results reveal the method ability to characterize ADCs primary structure in a comprehensive manner while reducing tremendously the number of experiments required. Data generated showed that sheathless CZE-ESI-MS/MS characteristics position the methodology developed as a relevant alternative for comprehensive multilevel characterization of these complex biomolecules.

Antibody-drug conjugates--an emerging class of cancer treatment

Antibody-drug conjugates (ADCs) are an emerging novel class of anticancer treatment agents that combines the selectivity of targeted treatment with the cytotoxic potency of chemotherapy drugs. New linker technology associated with novel highly potent cytotoxic payloads has permitted the development of more effective and safe ADCs. In recent years, two ADCs have been licensed, T-DM1 and brentuximab vedotin, and are already establishing their place in cancer treatment. A plethora of ADCs are being investigated in phases I and II trials, emerging data of which appears promising. As we deepen our understanding of what makes a successful ADC, an increasing number of ADCs will likely become viable treatment options as single agents or in combination with chemotherapy. This review will present the philosophy underlying ADCs, their main characteristics and current research developments with a focus on ADCs in solid tumours.

Antibody Drug Conjugates: Nonclinical Safety Considerations

Antibody drug conjugates (ADCs) are biopharmaceutical molecules consisting of a cytotoxic small molecule covalently linked to a targeted protein carrier via a stable cleavable or noncleavable linker. The process of conjugation yields a highly complex molecule with biochemical properties that are distinct from those of the unconjugated components. The impact of these biochemical differences on the safety and pharmacokinetic (PK) profile of the conjugate must be considered when determining the types of nonclinical safety studies required to support clinical development of ADCs. The hybrid nature of ADCs highlights the need for a science-based approach to safety assessment that incorporates relevant aspects of small and large molecule testing paradigms. This thinking is reflected in current regulatory guidelines, where sections pertaining to conjugates allow for a flexible approach to nonclinical safety testing. The aim of this article is to review regulatory expectations regarding early assessment of nonclinical safety considerations and discuss how recent advances in our understanding of ADC-mediated toxicity can be used to guide the types of nonclinical safety studies needed to support ADC clinical development. The review will also explore nonclinical testing strategies that can be used to streamline ADC development by assessing the safety and efficacy of next generation ADC constructs using a rodent screen approach.

Cancer nanoimmunotherapy using advanced pharmaceutical nanotechnology

Immunotherapy is a promising option for cancer treatment that might cure cancer with fewer side effects by primarily activating the host's immune system. However, the effect of traditional immunotherapy is modest, frequently due to tumor escape and resistance of multiple mechanisms. Pharmaceutical nanotechnology, which is also called cancer nanotechnology or nanomedicine, has provided a practical solution to solve the limitations of traditional immunotherapy. This article reviews the latest developments in immunotherapy and nanomedicine, and illustrates how nanocarriers (including micelles, liposomes, polymer-drug conjugates, solid lipid nanoparticles and biodegradable nanoparticles) could be used for the cellular transfer of immune effectors for active and passive nanoimmunotherapy. The fine engineering of nanocarriers based on the unique features of the tumor microenvironment and extra-/intra-cellular conditions of tumor cells can greatly tip the triangle immunobalance among host, tumor and nanoparticulates in favor of antitumor responses, which shows a promising prospect for nanoimmunotherapy.

Targeting tumor vasculature with aptamer-functionalized doxorubicin-polylactide nanoconjugates for enhanced cancer therapy

An A10 aptamer (Apt)-functionalized, sub-100 nm doxorubicin-polylactide (Doxo-PLA) nanoconjugate (NC) with controlled release profile was developed as an intravenous therapeutic strategy to effectively target and cytoreduce canine hemangiosarcoma (cHSA), a naturally occurring solid tumor malignancy composed solely of tumor-associated endothelium. cHSA consists of a pure population of malignant endothelial cells expressing prostate-specific membrane antigen (PSMA) and is an ideal comparative tumor model system for evaluating the specificity and feasibility of tumor-associated endothelial cell targeting by A10 Apt-functionalized NC (A10 NC). In vitro, A10 NCs were selectively internalized across a panel of PSMA-expressing cancer cell lines, and when incorporating Doxo, A10 Doxo-PLA NCs exerted greater cytotoxic effects compared to nonfunctionalized Doxo-PLA NCs and free Doxo. Importantly, intravenously delivered A10 NCs selectively targeted PSMA-expressing tumor-associated endothelial cells at a cellular level in tumor-bearing mice and dramatically increased the uptake of NCs by endothelial cells within the local tumor microenvironment. By virtue of controlled drug release kinetics and selective tumor-associated endothelial cell targeting, A10 Doxo-PLA NCs possess a desirable safety profile in vivo, being well-tolerated following high-dose intravenous infusion in mice, as supported by the absence of any histologic organ toxicity. In cHSA-implanted mice, two consecutive intravenous infusions of A10 Doxo-PLA NCs exerted rapid and substantial cytoreductive activities within a period of 7 days, resulting in greater than 70% reduction in macroscopic tumor-associated endothelial cell burden as a consequence of enhanced cell death and necrosis.

Antibody-Drug Conjugates: Design, Formulation and Physicochemical Stability

The convergence of advanced understanding of biology with chemistry has led to a resurgence in the development of antibody-drug conjugates (ADCs), especially with two recent product approvals. Design and development of ADCs requires the synergistic combination of the monoclonal antibody, the linker and the payload. Advances in antibody science has enabled identification and generation of high affinity, highly selective, humanized or human antibodies for a given target. Novel linker technologies have been synthesized and highly potent cytotoxic drug payloads have been created. As the first generation of ADCs utilizing lysine and cysteine chemistries moves through the clinic and into commercialization, second generation ADCs involving site specific conjugation technologies are being evaluated and tested. The latter aim to be better characterized and controlled, with wider therapeutic indices as well as improved pharmacokinetic-pharmacodynamic (PK-PD) profiles. ADCs offer some interesting physicochemical properties, due to conjugation itself, and to the (often) hydrophobic payloads that must be considered during their CMC development. New analytical methodologies are required for the ADCs, supplementing those used for the antibody itself. Regulatory filings will be a combination of small molecule and biologics. The regulators have put forth some broad principles but this landscape is still evolving.

Efficient in vivo antitumor effect of an immunotoxin based on ribotoxin α-sarcin in nude mice bearing human colorectal cancer xenografts

Tagging of RNases, such as the ribotoxin α-sarcin, with the variable domains of antibodies directed to surface antigens that are selectively expressed on tumor cells endows cellular specificity to their cytotoxic action. A recombinant single-chain immunotoxin based on the ribotoxin α-sarcin (IMTXA33αS), produced in the generally regarded as safe (GRAS) yeast Pichia pastoris, has been recently described as a promising candidate for the treatment of colorectal cancer cells expressing the glycoprotein A33 (GPA33) antigen, due to its high specific and effective cytotoxic effect on in vitro assays against targeted cells. Here we report the in vivo antitumor effectiveness of this immunotoxin on nude mice bearing GPA33-positive human colon cancer xenografts. Two sets of independent assays were performed, including three experimental groups: control (PBS) and treatment with two different doses of immunotoxin (50 or 100 μg/ injection) (n = 8). Intraperitoneal administration of IMTXA33αS resulted in significant dose-dependent tumor growth inhibition. In addition, the remaining tumors excised from immunotoxin-treated mice showed absence of the GPA33 antigen and a clear inhibition of angiogenesis and proliferative capacity. No signs of immunotoxin-induced pathological changes were observed from specimens tissues. Overall these results show efficient and selective cytotoxic action on tumor xenografts, combined with the lack of severe side effects, suggesting that IMTXA33αS is a potential therapeutic agent against colorectal cancer.

Large scale generation and characterization of anti-human IgA monoclonal antibody in ascitic fluid of BALB/c mice

PURPOSE

Monoclonal antibodies are potentially powerful tools used in biomedical research, diagnosis, and treatment of infectious diseases and cancers. The monoclonal antibody against Human IgA can be used as a diagnostic application to detect infectious diseases. The aim of this study was to improve an appropriate protocol for large-scale production of mAbs against IgA.

METHODS

For large-scale production of the monoclonal antibody, hybridoma cells that produce monoclonal antibodies against Human IgA were injected intraperitoneally into Balb/c mice that were previously primed with 0.5 ml Pristane. After ten days, ascitic fluid was harvested from the peritoneum of each mouse. The ELISA method was carried out for evaluation of the titration of produced mAbs. The ascitic fluid was investigated in terms of class and subclass by a mouse mAb isotyping kit. MAb was purified from the ascitic fluid by ion exchange chromatography. The purity of the monoclonal antibody was confirmed by SDS-PAGE, and the purified monoclonal antibody was conjugated with HRP.

RESULTS

Monoclonal antibodies with high specificity and sensitivity against Human IgA were prepared by hybridoma technology. The subclass of antibody was IgG1 and its light chain was the kappa type.

CONCLUSION

This conjugated monoclonal antibody could have applications in designing ELISA kits in order to diagnose different infectious diseases such as toxoplasmosis and H. Pylori.

Epidermal growth factor receptor-targeted immunoliposomes for delivery of celecoxib to cancer cells

Cyclooxygenase-2 (COX-2) is highly expressed in many different cancers. Therefore, the inhibition of the COX-2 pathway by a selective COX-2 inhibitor, celecoxib (CLX), may be an alternative strategy for cancer prevention and therapy. Liposomal drug delivery systems can be used to increase the therapeutic efficacy of CLX while minimizing its side effects. Previous studies have reported the encapsulation of CLX within the non-targeted long circulating liposomes and functional effect of these formulations against colorectal cancer cell lines. However, the selectivity and internalization of CLX-loaded liposomes can further be improved by grafting targeting ligands on their surface. Cetuximab (anti-epidermal growth factor receptor - EGFR - monoclonal antibody) is a promising targeting ligand since EGFR is highly expressed in a wide range of solid tumors. The aim of this study was to develop EGFR-targeted immunoliposomes for enhancing the delivery of CLX to cancer cells and to evaluate the functional effects of these liposomes in cancer cell lines. EGFR-targeted ILs, having an average size of 120nm, could encapsulate 40% of the CLX, while providing a sustained drug release profile. Cell association studies have also shown that the immunoliposome uptake was higher in EGFR-overexpressing cells compared to the non-targeted liposomes. In addition, the CLX-loaded-anti-EGFR immunoliposomes were significantly more toxic compared to the non-targeted ones in cancer cells with EGFR-overexpression but not in the cells with low EGFR expression, regardless of their COX-2 expression status. Thus, selective targeting of CLX with anti-EGFR immunoliposomes appears to be a promising strategy for therapy of tumors that overexpress EGFR.

Computational Construction of Antibody-Drug Conjugates Using Surface Lysines as the Antibody Conjugation Site and a Non-cleavable Linker

Antibody–drug conjugates (ADCs) constitute a category of anticancer targeted therapy that has gathered great interest during the last few years because of their potential to kill cancer cells while causing significantly fewer side effects than traditional chemotherapy. In this paper, a process of computational construction of ADCs is described, using the surface lysines of an antibody and a non-covalent linker molecule, as well as a cytotoxic substance, as files in Protein Data Bank format. Also, aspects related to the function, properties, and development of ADCs are discussed.

Development of novel ADCs: conjugation of tubulysin analogues to trastuzumab monitored by dual radiolabeling

Tubulysins are highly toxic tubulin-targeting agents with a narrow therapeutic window that are interesting for application in antibody-drug conjugates (ADC). For full control over drug-antibody ratio (DAR) and the effect thereof on pharmacokinetics and tumor targeting, a dual-labeling approach was developed, wherein the drug, tubulysin variants, and the antibody, the anti-HER2 monoclonal antibody (mAb) trastuzumab, are radiolabeled. (131)I-radioiodination of two synthetic tubulysin A analogues, the less potent TUB-OH (IC50 > 100 nmol/L) and the potent TUB-OMOM (IC50, ~1 nmol/L), and their direct covalent conjugation to (89)Zr-trastuzumab were established. Radioiodination of tubulysins was 92% to 98% efficient and conversion to N-hydroxysuccinimide (NHS) esters more than 99%; esters were isolated in an overall yield of 68% ± 5% with radiochemical purity of more than 99.5%. Conjugation of (131)I-tubulysin-NHS esters to (89)Zr-trastuzumab was 45% to 55% efficient, resulting in ADCs with 96% to 98% radiochemical purity after size-exclusion chromatography. ADCs were evaluated for their tumor-targeting potential and antitumor effects in nude mice with tumors that were sensitive or resistant to trastuzumab, using ado-trastuzumab emtansine as a reference. ADCs appeared stable in vivo. An average DAR of 2 and 4 conferred pharmacokinetics and tumor-targeting behavior similar to parental trastuzumab. Efficacy studies using single-dose TUB-OMOM-trastuzumab (DAR 4) showed dose-dependent antitumor effects, including complete tumor eradications in trastuzumab-sensitive tumors in vivo. TUB-OMOM-trastuzumab (60 mg/kg) displayed efficacy similar to ado-trastuzumab emtansine (15 mg/kg) yet more effective than trastuzumab. Our findings illustrate the potential of synthetic tubulysins in ADCs for cancer treatment.

Targeted gene suppression by inducing de novo DNA methylation in the gene promoter

Background

Targeted gene silencing is an important approach in both drug development and basic research. However, the selection of a potent suppressor has become a significant hurdle to implementing maximal gene inhibition for this approach. We attempted to construct a ‘super suppressor’ by combining the activities of two suppressors that function through distinct epigenetic mechanisms.

Results

Gene targeting vectors were constructed by fusing a GAL4 DNA-binding domain with a epigenetic suppressor, including CpG DNA methylase Sss1, histone H3 lysine 27 methylase vSET domain, and Kruppel-associated suppression box (KRAB). We found that both Sss1 and KRAB suppressors significantly inhibited the expression of luciferase and copGFP reporter genes. However, the histone H3 lysine 27 methylase vSET did not show significant suppression in this system. Constructs containing both Sss1 and KRAB showed better inhibition than either one alone. In addition, we show that KRAB suppressed gene expression by altering the histone code, but not DNA methylation in the gene promoter. Sss1, on the other hand, not only induced de novo DNA methylation and recruited Heterochromatin Protein 1 (HP1a), but also increased H3K27 and H3K9 methylation in the promoter.

Conclusions

Epigenetic studies can provide useful data for the selection of suppressors in constructing therapeutic vectors for targeted gene silencing.

MMP-2/9-oriented combinations enhance antitumor efficacy of EGFR/HER2-targeting fusion proteins and gemcitabine

To increase the antitumor efficacy, in the present study, we proposed several settings of matrix metalloproteinase (MMP)-2/9-oriented combinations that comprise the MMP-2/9-targeting fusion protein dFv-LDP and the MMP inhibitor doxycycline (DOX) in association with EGFR/HER2-bispecific fusion protein Ec-LDP-Hr, its enediyne-energized analogue Ec-LDP-Hr-AE, and gemcitabine (GEM). The expressions of various fusion proteins were detected by western blot analysis. Proliferation and migration inhibition of cells were determined by MTT and Transwell assay, respectively. The binding capability of dFv-LDP and Ec-LDP-Hr to cancer cells was examined by ELISA, cell immunofluorescence coimmunoprecipitation and confocal assays. Animal experiments were set to investigate the antitumor efficacy of various combinations against colorectal carcinoma HCT-15 xenograft in athymic mice. These two targeting proteins dFv-LDP and Ec-LDP-Hr had strong binding capabilities and antiproliferation effects on various cancer cell lines. Enhanced therapeutic efficacy in vivo was observed in the MMP-2/9-targeting fusion protein dFv-LDP integrated combinations including: i) dFv-LDP and Ec-LDP-Hr, ii) dFv-LDP and enediyne-energized fusion protein Ec-LDP-Hr-AE, iii) dFv-LDP and Ec-LDP-Hr-AE plus DOX, and iv) dFv-LDP and GEM plus DOX against colorectal cancer HCT-15 xenograft in athymic mice. In setting iii, DOX (20 mg/kg), dFv-LDP (20 mg/kg) and Ec-LDP-Hr-AE (0.3 mg/kg) alone suppressed tumor growth by 35, 49.7 and 67.5%, respectively. The combination of dFv-LDP and Ec-LDP-Hr-AE was 75.1%. Furthermore, this combination plus DOX showed stronger efficacy with an inhibitory rate of 82.7%. In setting iv, the combination of dFv-LDP and GEM suppressed tumor growth by 66.3%. Notably, the tumor inhibitory rate of the dFv-LDP/GEM/DOX combination reached 85.5%, producing initial shrinkage after the first administration. The MMP-2/9-oriented combination strategy that employs the MMP-2/9-targeting antibody-based fusion protein and the small molecular inhibitor DOX as the basic composed agents may enhance antitumor efficacy in association with the EGFR/HER2-targeting fusion protein and GEM. This multiple targeting approach may be useful for enhancing antitumor efficacy against colorectal cancer.

Convection enhanced delivery of different molecular weight tracers of gadolinium-tagged polylysine

Convection enhanced delivery (CED) is a powerful method of circumventing the blood-brain barrier (BBB) to deliver therapeutic compounds directly to the CNS. While inferring the CED distribution of a therapeutic compound by imaging a magnetic resonance (MR)-sensitive tracer has many advantages, however how the compound distribution is affected by the features of the delivery system, its target tissue, and its molecular properties, such as its binding characteristics, charge, and molecular weight (MW) are not fully understood. We used MR imaging of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA)-tagged polylysine compounds of various MW, in vitro and in vivo, to measure the dependence of compounds MW on CED distribution. For the in vitro studies, the correlation between volume of distribution (Vd) as a function of MW was determined by measuring the T1 of the infused tracers, into 0.6% agarose gels through a multiport catheter. The compounds distributed in the gels inversely proportional to their MW, consistent with convection and unobstructed diffusion through a porous media. For the in vivo studies, Gd-DTPA tagged compounds were infused into the non-human primate putamen, via an implanted multiport catheter connected to a MedStream™ pump, programmed to deliver a predetermined volume with alternating on-off periods to take advantage of the convective and diffusive contributions to Vd. Unlike the gel studies, the higher MW polylysine-tracer infusions did not freely distribute from the multiport catheter in the putamen, suggesting that distribution was impeded by other properties that should also be considered in future tracer design and CED infusion protocols.