Dynamic, nonsink method for the simultaneous determination of drug permeability and binding coefficients in liposomes

Investigating pharmacokinetic profiles of Centella asiatica using machine learning and PBPK modelling

Physiologically based pharmacokinetic (PBPK) modeling serves as a valuable tool for determining the distribution and disposition of substances in the body of an organism. It involves a mathematical representation of the interrelationships among crucial physiological, biochemical, and physicochemical parameters. A lack of the values of pharmacokinetic parameters can be challenging in constructing a PBPK model. Herein, we propose an artificial intelligence framework to evaluate a key pharmacokinetic parameter, the intestinal effective permeability (Peff). The publicly available Peff dataset was utilized to develop regression machine learning models. The XGBoost model demonstrates the best test accuracy of R-squared (R2, coefficient of determination) of 0.68. The model is then applied to compute the Peff of asiaticoside and madecassoside, the parent compounds found in Centella asiatica. Subsequently, PBPK modeling was conducted to evaluate the biodistribution of the herbal substances following oral administration in a rat model. The simulation results were evaluated and validated, which agreed with the existing in vivo studies in rats. This in silico pipeline presents a potential approach for investigating the pharmacokinetic parameters and profiles of drugs or herbal substances, which can be used independently or integrated into other modeling systems.

A study on attempt for determination of permeation kinetics of coumarin at lipid bilayer of liposomes by using capillary electrophoresis with moment analysis theory

It was tried to develop a moment analysis method for the determination of lipid membrane permeability. The first absolute and second central moments of elution peaks measured by liposome electrokinetic chromatography (LEKC) are analyzed by using moment equations. As a concrete example, elution peak profiles of coumarin in a LEKC system, in which liposomes consisting of 1-palmitoyl-2-oleoyl-sn‑glycero-3-phosphocholine (POPC) and phosphatidylserine (PS) are used as a pseudo-stationary phase, were analyzed. It seems that lipid membrane permeability of coumarin across the lipid bilayer of POPC/PS liposomes was measured by the moment analysis method because previous permeability measurements using parallel artificial membrane permeability assay (PAMPA) and Caco-2 cells indicated that coumarin is permeable across lipid bilayer. However, it was also pointed out that the moment analysis method with LEKC is not effective for the determination of lipid membrane permeability and that it provides information about adsorption/desorption kinetics at lipid bilayer of liposomes. Therefore, different moment equations were also developed for the determination of adsorption/desorption rate constants of coumarin from the LEKC data. It was demonstrated that permeation rate constants at lipid bilayer or adsorption/desorption rate constants can be determined from the LEKC data on the basis of moment analysis theory for the mass transfer phenomena of coumarin at the lipid bilayer of POPC/PS liposomes. Mass transfer kinetics of solutes at lipid bilayer should be determined under the conditions that liposomes originally be because they are self-assembling and dynamic systems formed through weak interactions between phospholipid monomers. The moment analysis method using LEKC is effective for the experimental determination of the mass transfer rate constants at the lipid bilayer of liposomes because neither immobilization nor chemical modification of liposomes is necessary when LEKC data are measured. It is expected that the results of this study contribute to the dissemination of an opportunity for the determination of permeation rate constants or adsorption/desorption rate constants at the lipid bilayer of liposomes to many researchers because capillary electrophoresis is widespread.

Novel Phenobarbital-Loaded Nanostructured Lipid Carriers for Epilepsy Treatment: From QbD to In Vivo Evaluation

Pharmacological treatments of central nervous system diseases are always challenging due to the restrictions imposed by the blood–brain barrier: while some drugs can effectively cross it, many others, some antiepileptic drugs among them, display permeability issues to reach the site of action and exert their pharmacological effects. The development of last-generation therapeutic nanosystems capable of enhancing drug biodistribution has gained ground in the past few years. Lipid-based nanoparticles are promising systems aimed to improve or facilitate the passage of drugs through biological barriers, which have demonstrated their effectiveness in various therapeutic fields, without signs of associated toxicity. In the present work, nanostructured lipid carriers (NLCs) containing the antiepileptic drug phenobarbital were designed and optimized by a quality by design approach (QbD). The optimized formulation was characterized by its entrapment efficiency, particle size, polydispersity index, and Z potential. Thermal properties were analyzed by DSC and TGA, and morphology and crystal properties were analyzed by AFM, TEM, and XRD. Drug localization and possible interactions between the drug and the formulation components were evaluated using FTIR. In vitro release kinetic, cytotoxicity on non-tumoral mouse fibroblasts L929, and in vivo anticonvulsant activity in an animal model of acute seizures were studied as well. The optimized formulation resulted in spherical particles with a mean size of ca. 178 nm and 98.2% of entrapment efficiency, physically stable for more than a month. Results obtained from the physicochemical and in vitro release characterization suggested that the drug was incorporated into the lipid matrix losing its crystalline structure after the synthesis process and was then released following a slower kinetic in comparison with the conventional immediate-release formulation. The NLC was non-toxic against the selected cell line and capable of delivering the drug to the site of action in an adequate amount and time for therapeutic effects, with no appreciable neurotoxicity. Therefore, the developed system represents a promising alternative for the treatment of one of the most prevalent neurological diseases, epilepsy.

Development of an In Vitro Release Assay for Low-Density Cannabidiol Nanoparticles Prepared by Flash NanoPrecipitation

Nanoparticle encapsulation is an attractive approach to improve the oral bioavailability of hydrophobic therapeutics. The high specific surface area of nanoparticle formulations, combined with the thermodynamically driven increased solubility of an amorphous drug core, promotes rapid drug dissolution. However, the physicochemical properties of the hydrophobic therapeutic can present obstacles to in vitro characterization of nanoparticle formulations. Namely, drugs with low density and high membrane binding affinity frustrate traditional analytical methods to monitor release kinetics from nanoparticles. In this work, cannabidiol (CBD) was encapsulated into nanoparticles with low polydispersity and high drug loading via Flash NanoPrecipitation (FNP), a scalable self-assembly process. Hydroxypropyl methylcellulose acetate succinate (HPMCAS) and lecithin were employed as amphiphilic particle stabilizers during the FNP process. However, the low density and high membrane binding affinity of the amorphous CBD nanoparticle core prevented the characterization of in vitro release kinetics by conventional methods. Released CBD could not be separated from intact nanoparticles by filtration or centrifugation. To address this challenge, an alternative approach is described to coencapsulate 6 nm hydrophobic Fe₃O₄ colloids with CBD during FNP. The Fe₃O₄ colloids were added at 33% by mass (approximately 20% by volume) to increase the density of the nanoparticles, resulting in particles with an average diameter of 160 nm (CBD-lecithin-Fe₃O₄) or 280 nm (CBD-HPMCAS-Fe₃O₄). This densification enabled the centrifugal separation of dissolved (released) CBD from unreleased CBD during the in vitro assay while avoiding the losses associated with a filtration step. The resulting nanoparticle formulations provided more rapid and complete in vitro dissolution kinetics than bulk CBD, representing a 6-fold improvement in dissolution compared to crystalline CBD. The coencapsulation of high-density Fe₃O₄ colloids to enable the separation of nanoparticles from release media is a novel approach to measuring in vitro release kinetics of nanoencapsulated low-density, hydrophobic drug molecules.

Pulmonary delivery of osimertinib liposomes for non-small cell lung cancer treatment: formulation development and in vitro evaluation

Osimertinib (OB) is a third-generation irreversible tyrosine kinase inhibitor targeting the epidermal growth factor receptor (EGFR), overexpressed in non-small cell lung cancer. Systemic administration of drug often results in poor drug levels at the primary tumor in the lungs and is associated with systemic side effects. In this study, we developed inhalable OB liposomes that can locally accumulate at the tumor site thereby limiting systemic toxicity. OB was loaded into liposomes via active and passive loading methods. The OB active liposomes achieved a higher encapsulation (78%) compared to passive liposomes (25%). The liposomes (passive and active) exhibited excellent aerosolization performance with an aerodynamic diameter of 4 µm and fine particle fraction of 82%. In H1975 cells, OB active and passive liposomes reduced IC₅₀ by 2.2 and 1.2-fold, respectively, compared to free drug. As the OB active liposomes demonstrated higher cytotoxicity compared to OB passive liposomes, they were further investigated for in vitro anti-cancer activity. The OB active liposomes inhibited tumor cell migration and colonization as determined by the scratch assay and clonogenic assay, respectively. Furthermore, the 3D spheroid studies showed that the liposomes were successful in inhibiting tumor growth. These results highlight the potential of OB liposomes to suppress lung cancer. Owing to these attributes, the inhalable OB liposomes can potentially promote better therapeutic outcomes with limited systemic toxicity.

A Screening Study for the Development of Simvastatin-Doxorubicin Liposomes, a Co-Formulation with Future Perspectives in Colon Cancer Therapy

An increasing number of studies published so far have evidenced the benefits of Simvastatin (SIM) and Doxorubicin (DOX) co-treatment in colorectal cancer. In view of this, the current study aimed to investigate the pharmaceutical development of liposomes co-encapsulating SIM and DOX, by implementing the Quality by Design (QbD) concept, as a means to enhance the antiproliferative effect of the co-formulation on C26 murine colon cancer cells co-cultured with macrophages. It is known that the quality profile of liposomes is dependent on the critical quality attributes (CQAs) of liposomes (drug entrapped concentration, encapsulation efficiency, size, zeta potential, and drug release profile), which are, in turn, directly influenced by various formulation factors and processing parameters. By using the design of experiments, it was possible to outline the increased variability of CQAs in relation to formulation factors and identify by means of statistical analysis the material attributes that are critical (phospholipids, DOX and SIM concentration) for the quality of the co-formulation. The in vitro studies performed on a murine colon cancer cell line highlighted the importance of delivering the optimal drug ratio at the target site, since the balance antiproliferative vs. pro-proliferative effects can easily be shifted when the molar ratio between DOX and SIM changes.

Gelation of the internal core of liposomes as a strategy for stabilization and modified drug delivery II. Theoretical analysis and modelling of in-vitro release experiments

PEG-DMA was incorporated in unilamellar liposomes. PEG-DMA crosslinking by photo-induced radical reaction transforms the liquid aqueous core of the liposome into a hydrogel. The molecular weight of PEG-DMA significantly influences both structural and release properties of these hybrid nanosystems, by affecting both membrane permeability and diffusional properties of the inner core. Release studies of 5-(6) carboxyfluorescein from Conventional Liposomes (CL) and Gel-in-Liposome (GiL) systems were carried out in a vertical Franz Diffusion Cell. A detailed transport model is proposed, aimed at describing the entire drug diffusive pathway from the vesicles' inner core, through the double-layer membrane, into the buffer solution in the donor chamber of the Franz Cell and from there to the receptor chamber, where withdrawals are performed to evaluate the released drug concentration. The model permits to give a quantitative estimate of the diffusional resistances offered by the inner core (liquid or gelled) and by the double-layer membrane for CLs and different GiLs systems. The theoretical analysis of experimental release data strongly supports the basic assumption that, by varying the molecular weight of PEG-DMA, a different arrangement of the polymer within the liposomal structure and a different interaction with the membrane occur. PEG₇₅₀-DMA decreases the transport resistance of the double layer membrane with respect to CLs, while PEG₄₀₀₀-DMA plays the opposite role. After gelation of the internal core, the diffusional resistance to drug transport inside GiLs becomes controlling, thus significantly slowing down drug release from these systems. Therefore, the combination of PEG-DMA with phospholipid vesicles appears an interesting strategy to develop sustained drug delivery systems.

Adventitial injection delivery of nano-encapsulated sirolimus (Nanolimus) to injury-induced porcine femoral vessels to reduce luminal restenosis

Endovascular therapy in peripheral intervention has grown exponentially in the past decade, but the issue of high restenosis rates in lower extremity arteries still persist. While drug-coated balloons (DCB) have been the device of choice, recent controversary regarding the long-term safety of paclitaxel have raised concern over current DCBs. In our study, we proposed that the direct injection of a sirolimus nanoliposomal formulation (Nanolimus) using a infusion catheter can attenuate inflammation response in injured vessels. In vitro characterization showed retention of the nanoliposomes size and detectable drug amount up to 336 days in storage. For in vivo study, four female, mixed breed swines were subjected to balloon injury of the femoral arteries before treatment with either injection of saline (n = 4) or Nanolimus (n = 12) using the Bullfrog catheter. Pharmacokinetic analysis demonstrated sustained sirolimus release in the arteries and undetectable systemic drug level at 28 days. Arteries treated with Nanolimus showed significant reduction in neointima area (0.2 ± 0.3 mm² vs 2.0 ± 1.2 mm², p < 0.01) and luminal stenosis (14.2 ± 7.2% vs. 67.7 ± 24.8%, p < 0.01) compared to controls. In summary, adventitial delivery of sirolimus using an infusion catheter is a feasible and safe method to reduce vascular restenosis.

Nanomedicines - Tiny particles and big challenges

After decades of research, nanotechnology has been used in a broad array of biomedical products including medical devices, drug products, drug substances, and pharmaceutical-grade excipients. But like many great achievements in science, there is a fine balance between the risks and opportunities of this new technology. Some materials and surface structures in the nanosize range can exert unexpected toxicities and merit a more detailed safety assessment. Regulatory agencies such as the United States Food and Drug Administration or the European Medicines Agency have started dealing with the potential risks posed by nanomaterials. Considering that a thorough characterization is one of the key aspects of controlling such risks this review presents the regulatory background of nanosafety assessment and provides some practical advice on how to characterize nanomaterials and drug formulations. Further, the challenges of how to maintain and monitor pharmaceutical quality through a highly complex production processes will be discussed.

Predicting drug release kinetics from nanocarriers inside dialysis bags

Dialysis methods are frequently used to determine the in vitro drug release kinetics of nanoparticle drug delivery systems. However, the need for the released drug to diffuse through the dialysis membrane delays its appearance in the sampling compartment. Thus, the apparent drug release data outside the dialysis bag typically does not match the desired release kinetics inside the bag adjacent to the nanocarriers. To address this issue, here we describe a simple approach to determine the actual drug release kinetics from nano drug carriers inside the dialysis bag from the experimental data measured from the sampling compartment. First, a calibration experiment is carried out to determine the diffusion barrier properties of the dialysis membranes. The apparent drug release profile of the nanocarrier is then determined using the dialysis method, and a mathematical model is applied to determine the actual drug release kinetics from the experimental data. The model was tested on DOXIL® (doxorubicin liposomes), and an excellent agreement was found between the predicted and measured drug concentration inside the dialysis membranes. By taking the barrier effects of dialysis membranes into consideration, our model independent of drug carrier not only enables the proper interpretation of the data from dialysis studies but also helps to evaluate the dialysis methodology applied to in vitro drug release assays.

How to measure release from nanosized carriers?

Novel drug delivery systems exhibit great potential in the formulation of poorly soluble compounds but have also been applied to reduce side effects of highly active drug molecules. Despite all efforts, there are only few technologies available to investigate the in vitro release of next-generation nanotherapeutics. In the following, different approaches for testing the drug release from nanoparticles in the fields of formulation development and quality control will be discussed. A variety of methods is available, starting from dialysis-based equipment, in situ measurements, flow-through devices and sample and separate setups. If possible, these methods should enable a more rapid formulation development and quality control of nanosized carriers as well as improve the prediction of in vivo performance and clinical outcomes.

Nanopharmaceuticals for wound healing - Lost in translation?

Today, many of the newly developed pharmaceuticals and medical devices take advantage of nanotechnology and with a rising incidence of chronic diseases such as diabetes and cardiovascular disease, the number of patients afflicted globally with non-healing wounds is growing. This has created a requirement for improved therapies and wound care. However, converting the strategies applied in early research into new products is still challenging. Many of them fail to comply with the market requirements. This review discusses the legal and scientific challenges in the design of nanomedicines for wound healing. Are they lost in translation or is there a new generation of therapeutics in the pipeline?

Cholesterol modulates the liposome membrane fluidity and permeability for a hydrophilic molecule

The effect of cholesterol (CHOL) content on the permeability and fluidity of dipalmitoylphosphatidylcholine (DPPC) liposome membrane was investigated. Liposomes encapsulating sulforhodamine B (SRB), a fluorescent dye, were prepared by reverse phase evaporation technique (REV) at various DPPC:CHOL molar ratios (from 100:0 to 100:100). The release kinetics of SRB was studied during 48 h in buffer (pH 7.4) containing NaCl at 37 °C. The DPPC:CHOL formulations were also characterized for their size, polydispersity index and morphology. Increasing CHOL concentration induced an increase in the mean liposomes size accompanying with a shape transition from irregular to nanosized, regular and spherical vesicles. The release kinetics of SRB showed a biphasic pattern; the release data was then analyzed using different mathematical models. On the overall, the SRB release was governed by a non-Fickian diffusion during the first period (0-10 h) while it followed a Fickian diffusion between 10 and 48 h. Changes in DPPC liposome membrane fluidity of various batches (CHOL% 0, 10, 20, 30 and 100) were monitored by using 5- and 16 doxyl stearic acids (DSA) as spin labels. CHOL induced a decrease in the bilayer fluidity. Concisely, CHOL represents a critical component in modulating the release of hydrophilic molecules from lipid vesicles.

Ion-Pairing Contribution to the Liposomal Transport of Topotecan as Revealed by Mechanistic Modeling

Actively loaded liposomal formulations of anticancer agents have been widely explored due to their high drug encapsulation efficiencies and prolonged drug retention. Mathematical models to predict and optimize drug loading and release kinetics from these nanoparticle formulations would be useful in their development and may allow researchers to tune release profiles. Such models must account for the driving forces as influenced by the physicochemical properties of the drug and the microenvironment, and the liposomal barrier properties. This study employed mechanistic modeling to describe the active liposomal loading and release kinetics of the anticancer agent topotecan (TPT). The model incorporates ammonia transport resulting in generation of a pH gradient, TPT dimerization, TPT lactone ring-opening and -closing interconversion kinetics, chloride transport, and transport of TPT-chloride ion-pairs to describe the active loading and release kinetics of TPT in the presence of varying chloride concentrations. Model-based predictions of the kinetics of active loading at varying loading concentrations of TPT and release under dynamic dialysis conditions were in reasonable agreement with experiments. These findings identify key attributes to consider in optimizing and predicting loading and release of liposomal TPT that may also be applicable to liposomal formulations of other weakly basic pharmaceuticals.

IN VITRO RELEASE KINETICS MODEL FITTING OF LIPOSOMES: AN INSIGHT

Liposomes are emerging cargoes for bioactive delivery owing to their widely accepted biocompatible and biodegradable nature. It is always a challenge to control the release of payload for effective delivery to the site of interest. Over the couple of decennia, mathematical modeling of release process is a need of time whether the drug remains in the circulation or reaches at the target site. For establishing a better in vitro - in vivo correlation, release kinetics models viz. Peppas, Higuchi, Weibull, Zero Order and First order including mechanistic models like All-or-None, Toroidal, and Biomembrane models etc. are continuously exploited to predict drug release profile. Most of these models rely on the diffusion equations based on the composition of liposomes and conditions of release. Here, we summarized the crucial reports exploring these models and associated interventions to know the underlying physicochemical release phenomenon. Such mathematical model fitting can be a promising approach to deduce release/delivery process to help in designing the safe and efficacious ("Smart") liposomes.

Novel polymeric micelles for drug delivery: Material characterization and formulation screening

A rising number of new chemical entities that exhibit only poor aqueous solubility are identified in drug discovery processes. Polymeric micelles composed of block copolymers (BP) facilitate the delivery of such lipophilic molecules in drug therapy. Consequently, a rational screening and selection procedure for novel BP was established. Further, the interplay of polymer structure, micelle formation and drug binding was studied. Therefore seven polymers (BP001 to BP007) were synthesized from different monomer compositions resulting in nanocarriers varying in surface decoration and lipophilicity. These polymers were characterized by H(1)-NMR and SEC. The molecular weight was ranging between 13 and 37kDa. The critical micelle concentration and micellar integrity in presence of human plasma were determined. Micelles were loaded with dexamethasone and characterized with regards to their size, morphology and surface charge. Polymeric micelles with a size of 49.21-236.37nm were obtained. A half-life of 11h was determined for five of the copolymers in presence of human plasma. Two nanocarrier formulations (BP006 and BP007) were exhibiting optimal micellar integrity in vitro and a modified release profile under biorelevant conditions. Strongest drug-polymer interaction was observed for nanocarrier compositions providing benzyl and carboxylic groups and were composed of BP006 and BP007.

An in vitro assessment of liposomal topotecan simulating metronomic chemotherapy in combination with radiation in tumor-endothelial spheroids

Low dose metronomic chemotherapy (LDMC) refers to prolonged administration of low dose chemotherapy designed to minimize toxicity and target the tumor endothelium, causing tumor growth inhibition. Topotecan (TPT) when administered at its maximum tolerated dose (MTD) is often associated with systemic hematological toxicities. Liposomal encapsulation of TPT enhances efficacy by shielding it from systemic clearance, allowing greater uptake and extended tissue exposure in tumors. Extended release of TPT from liposomal formulations also has the potential to mimic metronomic therapies with fewer treatments. Here we investigate potential toxicities of equivalent doses of free and actively loaded liposomal TPT (LTPT) and compare them to a fractionated low dose regimen of free TPT in tumor-endothelial spheroids (TES) with/without radiation exposure for a prolonged period of 10 days. Using confocal microscopy, TPT fluorescence was monitored to determine the accumulation of drug within TES. These studies showed TES, being more reflective of the in vivo tumor microenvironment, were more sensitive to LTPT in comparison to free TPT with radiation. More importantly, the response of TES to low-dose metronomic TPT with radiation was comparable to similar treatment with LTPT. This TES study suggests nanoparticle formulations designed for extended release of drug can simulate LDMC in vivo.

Mechanistic model and analysis of doxorubicin release from liposomal formulations

Reliable and predictive models of drug release kinetics in vitro and in vivo are still lacking for liposomal formulations. Developing robust, predictive release models requires systematic, quantitative characterization of these complex drug delivery systems with respect to the physicochemical properties governing the driving force for release. These models must also incorporate changes in release due to the dissolution media and methods employed to monitor release. This paper demonstrates the successful development and application of a mathematical mechanistic model capable of predicting doxorubicin (DXR) release kinetics from liposomal formulations resembling the FDA-approved nanoformulation DOXIL® using dynamic dialysis. The model accounts for DXR equilibria (e.g. self-association, precipitation, ionization), the change in intravesicular pH due to ammonia release, and dialysis membrane transport of DXR. The model was tested using a Box-Behnken experimental design in which release conditions including extravesicular pH, ammonia concentration in the release medium, and the dilution of the formulation (i.e. suspension concentration) were varied. Mechanistic model predictions agreed with observed DXR release up to 19h. The predictions were similar to a computer fit of the release data using an empirical model often employed for analyzing data generated from this type of experimental design. Unlike the empirical model, the mechanistic model was also able to provide reasonable predictions of release outside the tested design space. These results illustrate the usefulness of mechanistic modeling to predict drug release from liposomal formulations in vitro and its potential for future development of in vitro - in vivo correlations for complex nanoformulations.

Insights into accelerated liposomal release of topotecan in plasma monitored by a non-invasive fluorescence spectroscopic method

A non-invasive fluorescence method was developed to monitor liposomal release kinetics of the anticancer agent topotecan (TPT) in physiological fluids and subsequently used to explore the cause of accelerated release in plasma. Analyses of fluorescence excitation spectra confirmed that unencapsulated TPT exhibits a red shift in its spectrum as pH is increased. This property was used to monitor TPT release from actively loaded liposomal formulations having a low intravesicular pH. Mathematical release models were developed to extract reliable rate constants for TPT release in aqueous solutions monitored by fluorescence and release kinetics obtained by HPLC. Using the fluorescence method, accelerated TPT release was observed in plasma as previously reported in the literature. Simulations to estimate the intravesicular pH were conducted to demonstrate that accelerated release correlated with alterations in the low intravesicular pH. This was attributed to the presence of ammonia in plasma samples rather than proteins and other plasma components generally believed to alter release kinetics in physiological samples. These findings shed light on the critical role that ammonia may play in contributing to the preclinical/clinical variability and performance seen with actively-loaded liposomal formulations of TPT and other weakly-basic anticancer agents.

Release, partitioning, and conjugation stability of doxorubicin in polymer micelles determined by mechanistic modeling

PURPOSE

To better understand the mechanistic parameters that govern drug release from polymer micelles with acid-labile linkers.

METHODS

A mathematical model was developed to describe drug release from block copolymer micelles composed of a poly(ethylene glycol) shell and a poly(aspartate) core, modified with drug binding linkers for pH-controlled release [hydrazide (HYD), aminobenzoate-hydrazide (ABZ), or glycine-hydrazide (GLY)]. Doxorubicin (Dox) was conjugated to the block copolymers through acid-labile hydrazone bonds. The polymer drug conjugates were used to prepare three polymer micelles (HYD-M, ABZ-M, and GLY-M). Drug release studies were performed to identify the factors governing pH-sensitive release of Dox. The effect of prolonged storage of copolymer material on release kinetics was also observed.

RESULTS

Biphasic drug release kinetics were observed for all three micelle formulations. The developed model was able to quantify observed release kinetics upon the inclusion of terms for unconjugated Dox and two populations of conjugated Dox. Micelle/water partitioning of Dox was also incorporated into the model and found significant in all micelles under neutral conditions but reduced under acidic conditions. The drug binding linker played a major role in drug release as the extent of Dox release at specific time intervals was greater at pH 5.0 than at pH 7.4 (HYD-M > ABZ-M > GLY-M). Mathematical modeling was also able to correlate changes in release kinetics with the instability of the hydrazone conjugation of Dox during prolonged storage.

CONCLUSION

These results illustrate the potential utility of mechanistic modeling to better assess release characteristics intrinsic to a particular drug/nanoparticle system.

The role of pH and ring-opening hydrolysis kinetics on liposomal release of topotecan

The use of liposomal delivery systems for the treatment of cancer has been extensively researched because of their passive targeting to the vasculature of solid tumors. While their potential to provide prolonged retention and high drug encapsulation is desirable for anticancer agents, a mechanistic understanding is required to optimize and design liposomal drug delivery systems capable of controllable release tailored to tumor type and patient. Topotecan (TPT) is a topoisomerase I inhibitor that undergoes reversible, pH-sensitive ring-opening hydrolysis. TPT may benefit from liposomal formulation using active loading strategies to generate low intravesicular pH to prolong drug retention and increase drug encapsulation. This paper develops a mathematical model to describe TPT's permeability as a function of pH by accounting for the drug's ionization state, membrane binding, and ring-opening interconversion kinetics. Studies were conducted to determine the acid dissociation constant of TPT's phenolic -OH and interconversion kinetics between TPT's lactone and carboxylate forms. Using the constants determined from these studies and release studies conducted at varying pH, permeability coefficients and membrane binding constants for each species of TPT were determined. Based on this model, three permeable species were observed. Interestingly, the two most permeable species were zwitterionic forms of TPT, and the permeability of the lactone zwitterion was comparable to that of the neutral form of another camptothecin analogue. Furthermore, release was affected by based-catalyzed interconversion kinetics between TPT's lactone and carboxylate forms. At neutral pH, release was rate-limited by formation of the TPT lactone from the ring-opened carboxylate form. Based on these findings, the developed model describing liposomal release of TPT may be used in the future to evaluate and optimize loading and subsequent release of liposomal TPT formulations utilizing active loading strategies.