Discovery of a Novel Cabazitaxel Nanoparticle-Drug Conjugate (CRLX522) with Improved Pharmacokinetic Properties and Anticancer Effects Using a ß-Cyclodextrin-PEG Copolymer Based Delivery Platform.

Novel nanoparticle-drug conjugates (NDCs) containing diverse, clinically relevant anticancer drug payloads (docetaxel, cabazitaxel, and gemcitabine) were successfully generated and tested in drug discovery studies. The NDCs utilized structurally varied linkers that attached the drug payloads to a ß-cyclodextrin-PEG copolymer to form self-assembled nanoparticles. In vitro release studies revealed a diversity of release rates driven by linker structure-activity relationships (SARs). Improved in vivo pharmacokinetics (PK) for the cabazitaxel (CBTX) NDCs with glycinate-containing (1c) and hexanoate-containing linkers (2c) were demonstrated, along with high and sustained tumor levels (>168 h of released drug in tumor tissues). This led to potent efficacy and survival in both taxane- and docetaxel-resistant in vivo anticancer mouse efficacy models. Overall, the CBTX-hexanoate NDC 2c (CRLX522), demonstrated optimal and improved in vivo PK (plasma and tumor) and efficacy profile versus those of the parent drug, and the results support the potential therapeutic use of CRLX522 as a new anticancer agent.

Tumor Selective Silencing Using an RNAi-Conjugated Polymeric Nanopharmaceutical.

Small interfering RNA (siRNA) therapeutics have potential advantages over traditional small molecule drugs such as high specificity and the ability to inhibit otherwise "undruggable" targets. However, siRNAs have short plasma half-lives in vivo, can induce a cytokine response, and show poor cellular uptake. Formulating siRNA into nanoparticles offers two advantages: enhanced siRNA stability against nuclease degradation beyond what chemical modification alone can provide; and improved site-specific delivery that takes advantage of the enhanced permeability and retention (EPR) effect. Existing delivery systems generally suffer from poor delivery to tumors. Here we describe the formation and biological activity of polymeric nanopharmaceuticals (PNPs) based on biocompatible and biodegradable poly(lactic-co-glycolic acid) (PLGA) conjugated to siRNA via an intracellular cleavable disulfide linker (PLGA-siRNA). Additionally, these PNPs contain (1) PLGA conjugated to polyethylene glycol (PEG) for enhanced pharmacokinetics of the nanocarrier; (2) a cation for complexation of siRNA and charge compensation to avoid high negative zeta potential; and (3) neutral poly(vinyl alcohol) (PVA) to stabilize the PNPs and support the PEG shell to prevent particle aggregation and protein adsorption. The biological data demonstrate that these PNPs achieve prolonged circulation, tumor accumulation that is uniform throughout the tumor, and prolonged tumor-specific knockdown. PNPs employed in this study had no effect on body weight, blood cell count, serum chemistry, or cytokine response at doses >10 times the effective dose. PNPs, therefore, constitute a promising solution for achieving durable siRNA delivery and gene silencing in tumors.

The dendrimer paradox--high medical expectations but poor clinical translation.

This review was written with the intention to critically evaluate the status of dendrimers as drug carriers and find answers as to why this class of compounds has not translated into the clinic despite 40 years of research. Topics addressed and challenged are the current state of dendrimer synthesis, for example the importance for surface multifunctionality and internal functional groups. Large numbers of surface groups are deemed one of the advantages of dendrimers; however, only small amounts of drugs can be conjugated to the surface without altering the dendrimer's performance, for example its solubility. On the other hand, the rarely utilized feature of internal functionalities for drug conjugation would allow drug loading without altering the surface composition and therefore lead to improved carrier-to-active weight ratios, a major concern for industrial drug product development. Synthetic approaches resulting in truly multifunctional nanocarriers based on chemical conjugation are being discussed, involving orthogonal and 'click' chemistries. Random conjugation of drug, imaging agent, and targeting ligand to the surface of pre-existing dendrimers results in poorly-defined compound mixtures that are unlikely to pass regulatory revision and translate into the clinic. Similarly, using dendrimers for physical drug entrapment is an approach with little clinical future because alternative, low-cost carriers are available and have translated to the market. Finally, a case is being made to evaluate other dendritic polymers such as dendrons, dendrigrafts, hyperbranched polymers, and dendronized polymers for delivery applications. Non-spherical shapes and structural flexibility are features generally discussed in vector-based drug delivery applications and therefore criteria worthwhile to evaluate.

What nanomedicine in the clinic right now really forms nanoparticles?

Some researchers believe nanomedicine will revolutionize healthcare and medicine through transformative new therapeutic tools. Nanocarriers, utilized to transport actives to the target site, are constructed from a wide range of materials. Nanocarriers can be grouped into self-assembling (liposomes, micelles), processed (nanoparticles, emulsions), and chemically bound (dendrimers, silica-based carriers, carbon nanotubes) structures. A review of nanomedicines on the market and in clinical translation reveals that the vast majority is based on liposomes, polymeric micelles, and nanoparticles. The increasing presence of these novel nanomedicines raises the question what nanomedicines in the clinic right now really form nanoparticles, i.e., are improvements we see from nanomedicines structure-related or do they result from improved formulations? Do we even have sufficient data to address this question? The formation of nanocarriers is usually confirmed in vitro but little if any in vivo (let alone clinical) information is available. Given the large number of nanomedicines on the market and under clinical evaluation one clearly cannot expect to find a 'one size fits all' answer. Therefore, two case studies are discussed: the paclitaxel formulation Taxol® and its nanomedicine companions LEP-ETU (liposome), Genexol®-PM and NK105 (micelles), and Abraxane® (nanoparticle). Published pharmacokinetic data is utilized to find differences indicating nanocarrier delivery. The second case study involves structurally related camptothecin-polymer conjugates CRLX101 (nanoparticles) and XMT-1001 (prodrug). Structural differences are evaluated to discuss the different aggregation behavior. This opinion can only serve as first attempt to find a more general answer; clearly more data is needed from future studies.

Theranostics: are we there yet?

The U.S. National Institutes of Health through the National Cancer Institute (NCI) have been charged with the goal of eliminating death and suffering from cancer by the year 2015. In order to achieve this very ambitious goal, the development of novel nanotechnology-based devices and therapeutics that are capable of one or more clinically important functions is envisioned. There is great hope and expectation in the development of theranostic nanocarriers, which combine diagnostic and therapeutic agents in one entity. Main delivery approaches include prodrugs, liposomes, polymersomes, and polymeric micelles and nanoparticles. Diagnostic and therapeutic agents are physically entrapped or conjugated to the nanocarriers, or they are conjugated to carefully designed polymers which subsequently form nanocarriers. This focus discusses pros and cons of the different theranostic approaches and tries to answer the question which approach has the highest probability to translate into the clinic and benefit patients. Carefully designed polymers, conjugated with diagnostic and therapeutic agents, that either self-assemble or can be processed to form nanocarriers offer clear advantages over random physical entrapment or conjugation of these agents to existing nanocarriers. These polymers can optionally be fitted with terminal stabilizing or anchoring functionalities and a targeting ligand. However, the need for nanocarriers that are subjected to the enhanced permeability and retention (EPR) effect to carry ligands for active targeting still needs to be demonstrated. Thirty-seven of the 41 nanocarrier-based formulations that are on the market or are under investigation at different levels of clinical development rely on passive targeting. The answer to the title question, not surprisingly, can only be no, but very promising approaches are being developed that have the potential to translate into the clinic and meet regulatory requirements.

Preclinical to clinical development of the novel camptothecin nanopharmaceutical CRLX101.

Camptothecin (CPT) is a potent broad-spectrum anticancer agent that acts through inhibition of topoisomerase 1. Clinical development of CPT was unsuccessful due to poor drug solubility, insufficient in vivo stability of the active form, and toxicity. In order to address these issues, a polymeric nanoparticle comprised of cyclodextrin-poly(ethylene glycol) copolymer (CDP) conjugated to CPT (CRLX101) has been developed and Phase 2 clinical studies are ongoing. Camptothecin is conjugated to the polymer in its active form at 10-12 wt.% loading. CRLX101 self-assembles in solution into nanoparticles with an apparent solubility increase of >1000-fold as compared to the parent drug camptothecin. Preclinical studies exhibited CRLX101 pharmacokinetics superior to the parent drug. Drug concentration in tumor relative to plasma and other major organs is consistent with the enhanced permeation and retention (EPR) anticipated from a nanoparticle. Significant anti-tumor activity was observed that is superior when compared to irinotecan across a broad range of xenograft models. Pharmacokinetic data are consistent with the prolonged half-life and increased AUC. The CRLX101 preclinical and clinical data confirm that CDP can address not only solubility, formulation, toxicity, and pharmacokinetic challenges associated with administration of CPT, but more importantly, can impart unique biological properties, that enhance pharmacodynamics and efficacy of camptothecin.

Dendrimers as versatile platform in drug delivery applications.

About forty percent of newly developed drugs are rejected by the pharmaceutical industry and will never benefit a patient because of poor bioavailability due to low water solubility and/or cell membrane permeability. New delivery technologies could help to overcome this challenge. Nanostructures with uniform and well-defined particle size and shape are of eminent interest in biomedical applications because of their ability to cross cell membranes and to reduce the risk of premature clearance from the body. The high level of control over the dendritic architecture (size, branching density, surface functionality) makes dendrimers ideal carriers in these applications. Many commercial small molecule drugs with anticancer, anti-inflammatory, and antimicrobial activity have been successfully associated with dendrimers such as poly(amidoamine) (PAMAM), poly(propylene imine) (PPI or DAB) and poly(etherhydroxylamine) (PEHAM) dendrimers, either via physical interactions or through chemical bonding ('prodrug approach'). Targeted delivery is possible via targeting ligands conjugated to the dendrimer surface or via the enhanced permeability and retention (EPR) effect. The biocompatibility of dendrimers follows patterns known from other small particles. Cationic surfaces show cytotoxicity; however, derivatization with fatty acid or PEG chains, reducing the overall charge density and minimizing contact between cell surfaces and dendrimers, can reduce toxic effects.

Dendrimers for enhanced drug solubilization.

Approximately 40% of newly developed drugs are rejected by the pharmaceutical industry and will never benefit a patient because of low water solubility. Another 17% of launched drugs exhibit suboptimal performance for the same reason. Given the growing impact and need for drug delivery, a thorough understanding of delivery technologies that enhance the bioavailability of drugs is important. The high level of control over the dendritic architecture (size, branching density, surface functionality) makes dendrimers ideal excipients for enhanced solubility of poorly water-soluble drugs. Many commercial small-molecule drugs with anticancer, anti-inflammatory and antimicrobial activity have been formulated successfully with dendrimers, such as poly(amidoamine) (PAMAM), poly(propylene imine) (PPI or DAB) and poly(etherhydroxylamine) (PEHAM). Some dendrimers themselves show pharmaceutical activity in these three areas, providing the opportunity for combination therapy in which the dendrimers serve as the drug carrier and simultaneously as an active part of the therapy.

Dendrimers in biomedical applications--reflections on the field.

The formation of particulate systems with well-defined sizes and shapes is of eminent interest in certain medical applications such as drug delivery, gene transfection, and imaging. The high level of control possible over the architectural design of dendrimers; their size, shape, branching length/density, and their surface functionality, clearly distinguishes these structures as unique and optimum carriers in those applications. The bioactive agents may be encapsulated into the interior of the dendrimers or chemically attached/physically adsorbed onto the dendrimer surface, with the option of tailoring the carrier to the specific needs of the active material and its therapeutic applications. In this regard, the high density of exo-presented surface groups allows attachment of targeting groups or functionality that may modify the solution behavior or toxicity of dendrimers. Quite remarkably, modified dendrimers have been shown to act as nano-drugs against tumors, bacteria, and viruses. Recent successes in simplifying and optimizing the synthesis of dendrimers such as the 'lego' and 'click' approaches, provide a large variety of structures while at the same time reducing the cost of their production. The reflections on biomedical applications of dendrimers given in this review clearly demonstrate the potential of this new fourth major class of polymer architecture and indeed substantiate the high hopes for the future of dendrimers.

Facile and Efficient Synthesis of Bolaamphiphilic Tetraether Phosphocholines.

A facile synthesis of ether-linked bolaform phospholipids in good yields has been developed. Triflic acid catalyzed oxirane ring opening of benzyl-protected rac-glycidyl with long-chain 1,omega-alkanediols (n = 16, 20) produced 1,1'-diglycerol diethers in 80-90% yield. Double alkylation of the secondary hydroxy groups with 1-bromooctane or 1-bromodecane gave the corresponding benzyl-protected tetraethers in 66% yield. Hydrogenolysis of the benzyl groups in the presence of Pd/C (55-66% yield) followed by phosphorylation with 2 equiv of 2-chloro-2-oxo-1,3,2-dioxaphospholane and amination with excess trimethylamine produced the tetraether bolaform bisphosphocholines as white powders in approximately 75% yield. This approach provides a reliable and efficient method for preparing a wide variety of symmetrical bolaform phospholipids on a multigram scale.