Stimuli-sensitive polymer prodrug nanocarriers by reversible-deactivation radical polymerization

Polymer prodrugs are based on the covalent linkage of therapeutic molecules to a polymer structure which avoids the problems and limitations commonly encountered with traditional drug-loaded nanocarriers in which drugs are just physically entrapped (e.g., burst release, poor drug loadings). In the past few years, reversible-deactivation radical polymerization (RDRP) techniques have been extensively used to design tailor-made polymer prodrug nanocarriers. This synthesis strategy has received a lot of attention due to the possibility of fine tuning their structural parameters (e.g., polymer nature and macromolecular characteristics, linker nature, physico-chemical properties, functionalization, etc.), to achieve optimized drug delivery and therapeutic efficacy. In particular, adjusting the nature of the drug-polymer linker has enabled the easy synthesis of stimuli-responsive polymer prodrugs for efficient spatiotemporal drug release. In this context, this review article will give an overview of the different stimuli-sensitive polymer prodrug structures designed by RDRP techniques, with a strong focus on the synthesis strategies, the macromolecular architectures and in particular the drug-polymer linker, which governs the drug release kinetics and eventually the therapeutic effect. Their biological evaluations will also be discussed.

Combinatorial approaches of nanotherapeutics for inflammatory pathway targeted therapy of prostate cancer

Prostate cancer (PC) is the most prevalent male urogenital cancer worldwide. PC patients presenting an advanced or metastatic cancer succumb to the disease, even after therapeutic interventions including radiotherapy, surgery, androgen deprivation therapy (ADT), and chemotherapy. One of the hallmarks of PC is evading immune surveillance and chronic inflammation, which is a major challenge towards designing effective therapeutic formulations against PC. Chronic inflammation in PC is often characterized by tumor microenvironment alterations, epithelial-mesenchymal transition and extracellular matrix modifications. The inflammatory events are modulated by reactive nitrogen and oxygen species, inflammatory cytokines and chemokines. Major signaling pathways in PC includes androgen receptor, PI3K and NF-κB pathways and targeting these inter-linked pathways poses a major therapeutic challenge. Notably, many conventional treatments are clinically unsuccessful, due to lack of targetability and poor bioavailability of the therapeutics, untoward toxicity and multidrug resistance. The past decade witnessed an advancement of nanotechnology as an excellent therapeutic paradigm for PC therapy. Modern nanovectorization strategies such as stimuli-responsive and active PC targeting carriers offer controlled release patterns and superior anti-cancer effects. The current review initially describes the classification, inflammatory triggers and major inflammatory pathways of PC, various PC treatment strategies and their limitations. Subsequently, recent advancement in combinatorial nanotherapeutic approaches, which target PC inflammatory pathways, and the mechanism of action are discussed. Besides, the current clinical status and prospects of PC homing nanovectorization, and major challenges to be addressed towards the advancement PC therapy are also addressed.

A nanofiber based antiviral (TAF) prodrug delivery system

HIV and hepatitis B are two of the most prevalent viruses globally, and despite readily available preventive treatments unforgiving treatment regimens still exist, such as daily doses of medicine that are challenging to maintain especially in poorer countries. More advanced and longer-lasting delivery vehicles can potentially overcome this problem by reducing maintenance requirements and significantly increase access to medicine. Here, we designed a technology to control the delivery of an antiviral drug over a long timeframe via a nanofiber based delivery scaffold that is both easy to produce and use. An antiviral prodrug containing tenofovir alafenamide (TAF) was synthesized by initial conjugation to glycerol monomethacrylate followed by polymerization to form a diblock copolymer (pTAF) using reversible addition-fragmentation chain transfer (RAFT). In order to generate an efficient drug delivery system this copolymer was fabricated into an electrospun nanofiber (ESF) scaffold using blend electrospinning with poly(caprolactone) (PCL) as the carrier polymer. SEM images revealed that the pTAF-PCL ESFs were uniform with an average diameter of (787 ± 0.212 nm), while XPS analysis demonstrated that the pTAF was overrepresented at the surface of the ESFs. Additionally, the pTAF exhibited a sustained release profile over a 2 month period in human serum (HS), suggesting that these types of copolymer-based drugamers can be used in conjunction with electrospinning to produce long-lasting drug delivery systems.

A macrophage-targeted platform for extending drug dosing with polymer prodrugs for pulmonary infection prophylaxis

Pulmonary melioidosis is a bacterial disease with high morbidity and a mortality rate that can be as high as 40% in resource-poor regions of South Asia. This disease burden is linked to the pathogen's intrinsic antibiotic resistance and protected intracellular localization in alveolar macrophages. Current treatment regimens require several antibiotics with multi-month oral and intravenous administrations that are difficult to implement in under-resourced settings. Herein, we report that a macrophage-targeted polyciprofloxacin prodrug acts as a surprisingly effective pre-exposure prophylactic in highly lethal murine models of aerosolized human pulmonary melioidosis. A single dose of the polymeric prodrug maintained high lung drug levels and targeted an intracellular depot of ciprofloxacin to the alveolar macrophage compartment that was sustained over a period of 7 days above minimal inhibitory concentrations. This intracellular pharmacokinetic profile provided complete pre-exposure protection in a BSL-3 model with an aerosolized clinical isolate of Burkholderia pseudomallei from Thailand. This total protection was achieved despite the bacteria's relative resistance to ciprofloxacin and where an equivalent dose of pulmonary-administered ciprofloxacin was ineffective. For the first time, we demonstrate that targeting the intracellular macrophage compartment with extended antibiotic dosing can achieve pre-exposure prophylaxis in a model of pulmonary melioidosis. This fully synthetic and modular therapeutic platform could be an important therapeutic approach with new or re-purposed antibiotics for melioidosis prevention and treatment, especially as portable inhalation devices in high-risk, resource-poor settings.

Clinical applications of nanomedicines in lung cancer treatment

Lung cancer is the leading cause of cancer mortality worldwide. Owing to a lack of early-stage diagnosis, most lung cancers are detected in advanced stages, limiting the available therapeutic options. Moreover, extensive systemic chemotherapy of lung tumors is often associated with severe off-target toxicity and drug resistance of cancer cells, thus diminishing the outcomes of chemotherapy modalities. In this light, nanomedicines have opened an alternative avenue to develop more efficacious therapeutic platforms while addressing several current challenges. Clinical findings have revealed that nanomedicines improve the pharmacokinetics and biodistribution of the therapeutic agents while decreasing their systemic toxicity. This review provides an update on nanomedicines that have been clinically approved or are undergoing clinical trials for treatment of lung cancer. By discussing the clinical findings of the current nanoformulations, this review provides prospects for the development of more efficacious nanomedicines to improve the clinical outcomes of lung cancer treatment.

Poly(HPMA)–chlorambucil conjugate nanoparticles: facile fabrication and in vitro anti-cancer activity

An acid-sensitive poly(HPMA)–Chl conjugate was developed and its antitumor effect towards HepG2 and MCF-7 cells was evaluated.

Fully synthetic injectable depots with high drug content and tunable pharmacokinetics for long-acting drug delivery

Clinical studies have validated that antiretroviral (ARV) drugs can serve as an HIV pre-exposure prophylactic (PrEP) strategy. Dosing adherence remains a crucial factor determining the final efficacy outcomes, and both long-acting implants and injectable depot systems are being developed to improve patient adherence. Here, we describe an injectable depot platform that exploits a new mechanism for both formation and controlled release. The depot is a polymeric prodrug synthesized from monomers that incorporate an ARV drug tenofovir alafenamide (TAF) with degradable linkers that can be designed to control release rates. The prodrug monomers are synthetically incorporated into homopolymer or block designs that exhibit high drug weight percent (wt%) and also are hydrophobized in these prodrug segments to drive depot formation upon injection. Drug release converts those monomers to more hydrophilic pendant groups via linker cleavage, and as this drug release proceeds, the polymer chains losing hydrophobicity are then disassociated from the depot and released over time to provide a depot dissolution mechanism. We show that long-acting TAF depots can be designed as block copolymers or as homopolymers. They can also be designed with different linkers, for example with faster or slower degrading p-hydroxybenzyloxycarbonyl (Benzyl) and ethyloxycarbonyl (Alkyl) linkers, respectively. Diblock designs of p(glycerol monomethacrylate)-b-p(Alkyl-TAF-methacrylate) and p(glycerol monomethacrylate)-b-p(Benzyl-TAF-methacrylate) were first characterized in a mouse subcutaneous injection model. The alkylcarbamate linker design (TAF 51 wt%) showed excellent sustained release profiles of the key metabolite tenofovir (TFV) in skin and plasma over a 50-day period. Next, the homopolymer design with a high TAF drug wt% of 73% was characterized in the same model. The homopolymer depots with p(Alkyl-TAFMA) exhibited sustained TFV and TAF release profiles in skin and blood over 60 days, and TFV-DP concentrations in peripheral blood mononuclear cells (PBMC) were found to be at least 10-fold higher than the clinically suggested minimally EC90 protective concentration of 24 fmol/10⁶ cells. These are the first reports of sustained parent TAF dosing observed in mouse and TFV-DP in mouse PBMC. IVIS imaging of rhodamine labeled homopolymer depots showed that degradation and release of the depot coincided with the sustained TAF release. Finally, these polymers showed excellent stability in accelerated stability studies over a six-month time period, and exceptional solubility of over 700 mg/mL in the DMSO formulation solvent. The homopolymer designs have a drug reservoir potential of well over a year at mg/day dosing and may not require cold chain storage for global health and developed world long-acting drug delivery applications.

Dual-Responsive Polyprodrug Nanoparticles with Cascade-Enhanced Magnetic Resonance Signals for Deep-Penetration Drug Release in Tumor Therapy

Smart transformable nanocarriers are promising to treat deep-seated diseases but require adaptable diagnostic/imaging potency to reflect the morphology change and therapeutic feedback, yet their design and synthesis remains challenging. Herein, stimuli-responsive polyprodrug nanoparticles (SPNs) are formulated from the co-assembly of negatively charged corona and positively charged polyprodrug cores, exhibiting high loading content of camptothecin (CPT, ∼28.6 wt %) tethered via disulfide linkages in the core. SPNs are sequentially sensitive to tumor acidic condition and elevated reductive milieu in the cytosol for deep-penetration drug delivery. Upon accumulation at acidic tumor sites, SPNs dissociate to release smaller positively charged polyprodrug nanoparticles, which efficiently enter deep-seated tumor cells to trigger high-dosage parent CPT release in the reductive cytosolic milieu. Meanwhile, the polyprodrug cores of SPNs labeled with DTPA(Gd), a magnetic resonance imaging contrast agent, can trace the cascade degradation and biodistribution of SPNs as well as the resulting intracellular CPT release. The longitudinal relaxivity of SPNs increases stepwise in the above two processes. The size-switchable polyprodrug nanoparticles exhibit remarkable tumor penetration and noteworthy tumor inhibition in vitro and in vivo, which are promising for endogenously activated precision diagnostics and therapy.

Parallel evolution of polymer chemistry and immunology: Integrating mechanistic biology with materials design

To develop new therapeutics involves the interaction of multiple disciplines to yield safe, functional devices and formulations. Regardless of drug function and potency, administration with controlled timing, dosing, and targeting is required to properly treat or regulate health and disease. Delivery approaches can be optimized through advances in materials science, clinical testing, and basic biology and immunology. Presently, laboratories focused on developing these technologies are composed of, or collaborate with, chemists, biologists, materials scientists, engineers, and physicians to understand the way our body interacts with drug delivery devices, and how to synthesize new, rationally designed materials to improve targeted and controlled drug delivery. In this review, we discuss both device-based and micro/nanoparticle-based materials in the clinic, our biologic understanding of how our immune system interacts with these materials, how this diverse set of immune cells has become a target and variable in drug delivery design, and new directions in polymer chemistry to address these interactions and further our advances in medical therapeutics.

Aromatic Nitrogen Mustard-Based Autofluorescent Amphiphilic Brush Copolymer as pH-Responsive Drug Delivery Vehicle

Delivery of clinically approved nonfluorescent drugs is facing challenges because it is difficult to monitor the intracellular drug delivery without incorporating any integrated fluorescence moiety into the drug carrier. The present investigation reports the synthesis of a pH-responsive autofluorescent polymeric nanoscaffold for the administration of nonfluorescent aromatic nitrogen mustard chlorambucil (CBL) drug into the cancer cells. Copolymerization of poly(ethylene glycol) (PEG) appended styrene and CBL conjugated N-substituted maleimide monomers enables the formation of well-defined luminescent alternating copolymer. These amphiphilic brush copolymers self-organized in aqueous medium into 25-68 nm nanoparticles, where the CBL drug is enclosed into the core of the self-assembled nanoparticles. In vitro studies revealed ∼70% drug was retained under physiological conditions at pH 7.4 and 37 °C. At endolysosomal pH 5.0, 90% of the CBL was released by the pH-induced cleavage of the aliphatic ester linkages connecting CBL to the maleimide unit. Although the nascent nanoparticle (without drug conjugation) is nontoxic, the drug conjugated nanoparticle showed higher toxicity and superior cell killing capability in cervical cancer (HeLa) cells rather than in normal cells. Interestingly, the copolymer without any conventional chromophore exhibited photoluminescence under UV light irradiation due to the presence of "through-space" π-π interaction between the C═O group of maleimide unit and the adjacent benzene ring of the styrenic monomer. This property helped us intracellular tracking of CBL conjugated autofluorescent nanocarriers through fluorescence microscope imaging. Finally, the 4-(4-nitrobenzyl)pyridine (NBP) colorimetric assay was executed to examine the ability of CBL-based polymeric nanomaterials toward alkylation of DNA.

Fully synthetic macromolecular prodrug chemotherapeutics with EGFR targeting and controlled camptothecin release kinetics

Herein, we developed a fully polymerizable, peptide-targeted, camptothecin polymeric prodrug system. Two prodrug monomers were synthesized via esterification of campothecin (20Cam) and 10-hydroxycamptothecin (10Cam) with mono-2-(methacryloyloxy)ethyl succinate (SMA) resulting in polymerizable forms of the aliphatic ester- and aromatic ester-linked drugs respectively. These monomers were then incorporated into zwitterionic polymers via RAFT copolymerization of the prodrug monomers with a tert-butyl ester protected carboxy betaine monomer. Subsequent deprotection of the tert-butyl residues with TFA yielded carboxy betaine methacrylate (CBM) scaffolds with controlled prodrug incorporation. Reverse phase HPLC was then employed to establish drug release kinetics in human serum at 37 oC for the resultant polymeric prodrugs. Copolymers containing 10Cam residues linked via aromatic esters showed faster hydrolysis rates with 59 % drug released at 7 days, while copolymers with Cam residues linked via aliphatic esters showed only 28 % drug release over the same time period. These differences in drug release kinetics were then shown to correlate with large differences in cytotoxic activity in SKOV3 ovarian cancer cell cultures. At 72 hours, the IC50s of aromatic- and aliphatic- ester linked prodrugs were 56 nM and 4776 nM, respectively. An EGFR-targeting peptide sequence, GE11, was then directly incorporated into the polymeric prodrugs via RAFT copolymerization of the polymeric prodrugs with a peptide macronomer. The GE11-targeted polymeric prodrugs showed enhanced targeting and cytotoxic activity in SKOV3 cell cultures relative to untargeted polymers containing the negative control sequence HW12. Following pulse-chase treatment (15 min, 37 °C), the 72 hour IC50 of GE11 targeted prodrug was determined to be 1597 nM, in contrast to 3399 nM for the non-targeted control.

Physicochemical Characterization of FRET-Labelled Chitosan Nanocapsules and Model Degradation Studies

Sub-micron o/w emulsions coated with chitosan have been used for drug delivery, quorum sensing inhibition, and vaccine development. To study interactions with biological systems, nanocapsules have been fluorescently labelled in previous works, but it is often difficult to distinguish the released label from intact nanocapsules. In this study, we present advanced-labelling strategies based on Förster Resonance Energy Transfer (FRET) measurements for chitosan-coated nanocapsules and investigate their dissolution and degradation. We used FRET measurements of nanocapsules loaded with equimolar concentrations of two fluorescent dyes in their oily core and correlated them with dynamic light scattering (DLS) count rate measurement and absorbance measurements during their disintegration by dissolution. Using count rate measurements, we also investigated the enzymatic degradation of nanocapsules using pancreatin and how protein corona formation influences their degradation. Of note, nanocapsules dissolved in ethanol, while FRET decreased simultaneously with count rate, and absorbance was caused by nanocapsule turbidity, indicating increased distance between dye molecules after their release. Nanocapsules were degradable by pancreatin in a dose-dependent manner, and showed a delayed enzymatic degradation after protein corona formation. We present here novel labelling strategies for nanocapsules that allow us to judge their status and an in vitro method to study nanocapsule degradation and the influence of surface characteristics.

Radiant star nanoparticle prodrugs for the treatment of intracellular alveolar infections

Radiant star nanoparticle prodrugs were synthesized in a two-step process by first homopolymerizing RAFT transmers followed by copolymerization from the hyperbranched polymer core.

Poly(N-(2-Hydroxypropyl) Methacrylamide)-Valproic Acid Conjugates as Block Copolymer Nanocarriers

We report nanoassemblies based on block copolymers of N-(2-hydroxypropyl) methacrylamide (HPMA) in which drug cleavage enhances the biological compatibility of the original polymer carrier by regeneration of HPMA units. Drug release via ester hydrolysis suggests this approach offers potential for stimuli-responsive drug delivery under acidic conditions.

Doxorubicin delivered by a redox-responsive dasatinib-containing polymeric prodrug carrier for combination therapy

Two novel prodrug polymers POEG-b-PSSDas (redox-sensitive) and POEG-b-PCCDas (redox-insensitive), which consist of poly(oligo(ethylene glycol) methacrylate) (POEG) hydrophilic blocks and dasatinib (DAS, an oncogenic tyrosine kinases inhibitor) conjugated hydrophobic blocks, were designed as dual-functional carriers for codelivery with doxorubicin (DOX). Both carriers retained antitumor activity of DAS and could form mixed micelles with DOX. Compared to POEG-b-PCCDas micelles, incorporation of disulfide linkage into POEG-b-PSSDas micelles facilitated efficient cleavage of DAS from prodrug micelles in tumor cells/tissues, leading to a higher level of anti-tumor activity in vitro and in vivo. In addition, DOX-loaded POEG-b-PSSDas micelles exhibited triggered DOX release under a redox environment (10mM glutathione, GSH), and demonstrated enhanced cytotoxicity against 4T1.2 and PC3 cell lines compared to DOX and DOX-loaded POEG-b-PCCDas micelles. More importantly, DOX-loaded POEG-b-PSSDas micelles were more effective in inhibiting the tumor growth and prolonging the survival rate in an aggressive murine breast cancer model (4T1.2) compared to DOX-loaded POEG-b-PCCDas micelles and a micellar formulation co-loaded with DOX and DAS. This redox-responsive prodrug micellar system provides an attractive strategy for effective combination of tumor targeted therapy and traditional chemotherapy, which warrants further investigation.

Development of a Gemcitabine-Polymer Conjugate with Prolonged Cytotoxicity against a Pancreatic Cancer Cell Line

Gemcitabine (GEM) is a nucleoside analogue of deoxycytidine with limited therapeutic efficacy due to enzymatic hydrolysis by cytidine deaminase (CDA) resulting in compromised half-life in the bloodstream and poor pharmacokinetics. To overcome these limitations, we have developed a methacrylate-based GEM-monomer conjugate, which was polymerized by reversible addition-fragmentation chain transfer (RAFT) polymerization with high monomer conversion (∼90%) and low dispersity (<1.4). The resulting GEM-polymer conjugates were found to form well-defined sub-90 nm nanoparticles (NPs) in aqueous suspension. Subsequently, the GEM release was studied at different pH (∼7 and ∼5) with and without the presence of an enzyme, Cathepsin B. The GEM release profiles followed a pseudo zero-order rate and the GEM-polymer conjugate NPs were prone to acidic and enzymatic degradation, following a two-step hydrolysis mechanism. Furthermore, the NPs exhibited significant cytotoxicity in vitro against a model pancreatic cell line. Although, the half-maximal inhibitory concentration (IC₅₀) of the GEM-monomer and -polymer conjugate NPs was higher than free GEM, the conjugates showed superiorly prolonged activity compared to the parent drug.

Synthetic Macromolecular Antibiotic Platform for Inhalable Therapy against Aerosolized Intracellular Alveolar Infections

Lung-based intracellular bacterial infections remain one of the most challenging infectious disease settings. For example, the current standard for treating Franciscella tularensis pneumonia (tularemia) relies on administration of oral or intravenous antibiotics that poorly achieve and sustain pulmonary drug bioavailability. Inhalable antibiotic formulations are approved and in clinical development for upper respiratory infections, but sustained drug dosing from inhaled antibiotics against alveolar intracellular infections remains a current unmet need. To provide an extended therapy against alveolar intracellular infections, we have developed a macromolecular therapeutic platform that provides sustained local delivery of ciprofloxacin with controlled dosing profiles. Synthesized using RAFT polymerization, these macromolecular prodrugs characteristically have high drug loading (16-17 wt % drug), tunable hydrolysis kinetics mediated by drug linkage chemistry (slow-releasing alkyllic vs fast-releasing phenolic esters), and, in general, represent new fully synthetic nanotherapeutics with streamlined manufacturing profiles. In aerosolized and completely lethal F.t. novicida mouse challenge models, the fast-releasing ciprofloxacin macromolecular prodrug provided high cure efficiencies (75% survival rate under therapeutic treatment), and the importance of release kinetics was demonstrated by the inactivity of the similar but slow-releasing prodrug system. Pharmacokinetics and biodistribution studies further demonstrated that the efficacious fast-releasing prodrug retained drug dosing in the lung above the MIC over a 48 h period with corresponding C_max/MIC and AUC_0-24h/MIC ratios being greater than 10 and 125, respectively; the thresholds for optimal bactericidal efficacy. These findings identify the macromolecular prodrug platform as a potential therapeutic system to better treat alveolar intracellular infections such as F. tularensis, where positive patient outcomes require tailored antibiotic pharmacokinetic and treatment profiles.

Synthesis of zwitterionic, hydrophobic, and amphiphilic polymers via RAFT polymerization induced self-assembly (PISA) in acetic acid

Hydrophilic, hydrophobic, and combinations of these monomers were directly (co)polymerized via RAFT polymerization induced self-assembly (PISA) in acetic acid.

Combination anti-HIV therapy via tandem release of prodrugs from macromolecular carriers

Reversible addition-fragmentation chain transfer (RAFT) polymerisation has been used to create a library of copolymers outfitted with a combination of self-immolative reverse transcriptase inhibitor prodrug pendents comprising zidovudine (AZT) and lamivudine (3TC).