Polymer–drug conjugates for novel molecular targets

Polymer therapeutics-prospects for 21st century: the end of the beginning

The term "polymer therapeutics" was coined to describe polymeric drugs, polymer conjugates of proteins, drugs and aptamers, together with those block copolymer micelles and multicomponent non-viral vectors which contain covalent linkages. These often complex, multicomponent constructs are actually "drugs" and "macromolecular prodrugs" in contrast to drug delivery systems that simply entrap (non-covalently) therapeutic agents. They have also been described as nanomedicines. First polymer-protein conjugates entered routine clinical use in 1990 and a growing number of polymeric drugs/sequestrants and PEGylated proteins/aptamers have since come into the market. Valuable lessons have been learnt over >3 decades of clinical development, especially in relation to critical product attributes governing safety and efficacy, the validated methods needed for product characterisation. Not least there has been improved understanding of polymer therapeutic-specific biomarkers that will in future enable improved selection of patients for therapy. Advances in synthetic polymer chemistry (including control of 3D architecture), the move towards greater use of biodegradable polymers, polymers delivering combination therapy, increased understanding of polymer therapeutic critical product attributes to guide pharmaceutical development, and advances in understanding of endocytosis and intracellular trafficking pathways in health and disease are opening new opportunities for design and clinical use of polymer-based therapeutics in the decades to come.

Administration, distribution, metabolism and elimination of polymer therapeutics

Polymer conjugation is an efficient approach to improve the delivery of drugs and biological agents, both by protecting the body from the drug (by improving biodistribution and reducing toxicity) and by protecting the drug from the body (by preventing degradation and enhancing cellular uptake). This review discusses the journey that polymer therapeutics make through the body, following the ADME (absorption, distribution, metabolism, excretion) concept. The biological factors and delivery system parameters that influence each stage of the process will be described, with examples illustrating the different solutions to the challenges of drug delivery systems in vivo.

Targeted polymeric therapeutic nanoparticles: design, development and clinical translation

Polymeric materials have been used in a range of pharmaceutical and biotechnology products for more than 40 years. These materials have evolved from their earlier use as biodegradable products such as resorbable sutures, orthopaedic implants, macroscale and microscale drug delivery systems such as microparticles and wafers used as controlled drug release depots, to multifunctional nanoparticles (NPs) capable of targeting, and controlled release of therapeutic and diagnostic agents. These newer generations of targeted and controlled release polymeric NPs are now engineered to navigate the complex in vivo environment, and incorporate functionalities for achieving target specificity, control of drug concentration and exposure kinetics at the tissue, cell, and subcellular levels. Indeed this optimization of drug pharmacology as aided by careful design of multifunctional NPs can lead to improved drug safety and efficacy, and may be complimentary to drug enhancements that are traditionally achieved by medicinal chemistry. In this regard, polymeric NPs have the potential to result in a highly differentiated new class of therapeutics, distinct from the original active drugs used in their composition, and distinct from first generation NPs that largely facilitated drug formulation. A greater flexibility in the design of drug molecules themselves may also be facilitated following their incorporation into NPs, as drug properties (solubility, metabolism, plasma binding, biodistribution, target tissue accumulation) will no longer be constrained to the same extent by drug chemical composition, but also become in-part the function of the physicochemical properties of the NP. The combination of optimally designed drugs with optimally engineered polymeric NPs opens up the possibility of improved clinical outcomes that may not be achievable with the administration of drugs in their conventional form. In this critical review, we aim to provide insights into the design and development of targeted polymeric NPs and to highlight the challenges associated with the engineering of this novel class of therapeutics, including considerations of NP design optimization, development and biophysicochemical properties. Additionally, we highlight some recent examples from the literature, which demonstrate current trends and novel concepts in both the design and utility of targeted polymeric NPs (444 references).

Defining and designing polymers and hydrogels for neural tissue engineering

The use of biomaterials, such as hydrogels, as neural cell delivery devices is becoming more common in areas of research such as stroke, traumatic brain injury, and spinal cord injury. When reviewing the available research there is some ambiguity in the type of materials used and results are often at odds. This review aims to provide the neuroscience community who may not be familiar with fundamental concepts of hydrogel construction, with basic information that would pertain to neural tissue applications, and to describe the use of hydrogels as cell and drug delivery devices. We will illustrate some of the many tunable properties of hydrogels and the importance of these properties in obtaining reliable and consistent results. It is our hope that this review promotes creative ideas for ways that hydrogels could be adapted and employed for the treatment of a broad range of neurological disorders.