Biopharmaceutical considerations in topical ocular drug delivery

1. Despite the accessibility of the front of the eye, efficient delivery of drug to treat various ocular disorders is a challenge to the formulation scientist. The majority of ophthalmic medications are formulated as eye drops. Due to anatomical constraints, the volume that can be administered is limited to approximately 30 microL. This, together with the efficient clearance system that exists in the front of the eye, makes it difficult to maintain an effective pre-ocular drug concentration for a desired length of time. Various formulation strategies have been used to increase pre-ocular retention of eye drops. The most successful of these has been the inclusion of viscosity enhancing polymers, particularly those able to interact with the mucous layer on the eye surface or those that can undergo a transition from a solution to a gel under the conditions of the pre-ocular area. 2. When the target site is intra-ocular, drug must be absorbed from the pre-ocular region into the eye. The main route for absorption is across the cornea. However, absorption of drug across the cornea is inefficient due to its impermeable nature and small surface area. Thus, the intra-ocular bioavailability of topically administered medications is typically less than 10%. 3. Corneal permeability favours moderately lipophilic compounds. These compounds often have a low aqueous solubility. Problems in ocular drug delivery and formulation are compounded for poorly soluble drugs that must be formulated as suspensions. 4. Reformulation of ophthalmic suspensions as solutions has many advantages. This may be achieved by complexation using cyclodextrins. Solubilization using cyclodextrins can overcome many of the formulation problems. However, it is unclear as to their potential for improving ocular bioavailability, which is seemingly drug dependent and may be influenced by both the physicochemical properties of the drug and the complex formed.     

Biopharmaceutical considerations on antihistamine effects of topically administered emedastine

Antihistamine effects of emedastine applied topically with three vehicles varying in their polarities were investigated in rats. The pharmacological effect of emedastine differed greatly depending on its concentration, treatment time, and vehicle. The antihistamine effect reached a plateau after approximately 2 h of exposure, and the potency of emedastine decreased in the following order by vehicle: isopropyl myristate >geraniol >glycerin. Using a two-layer diffusion model and penetration parameters reported previously (Harada et al., Biol Pharm Bull 23:1224-1228, 2000), transdermal fluxes of emedastine at each time point were calculated. When antihistamine effect of emedastine was plotted against calculated in vitro transdermal flux, not concentration applied, their relationship was sigmoidal and common regardless of the vehicles used. In conclusion, the antihistamine effect of emedastine applied topically varied greatly depending on vehicle, fundamentally due to the difference in skin permeability. The transdermal flux of the drug appears to be a good measure of its pharmacological effect.     

Freeze-drying of nanoparticles: formulation, process and storage considerations

Freeze-drying has been considered as a good technique to improve the long-term stability of colloidal nanoparticles. The poor stability in an aqueous medium of these systems forms a real barrier against the clinical use of nanoparticles. This article reviews the state of the art of freeze-drying nanoparticles. It discusses the most important parameters that influence the success of freeze-drying of these fragile systems, and provides an overview of nanoparticles freeze-drying process and formulation strategies with a focus on the impact of formulation and process on particle stability.     

DC-SIGN mediated antigen-targeting using glycan-modified liposomes: formulation considerations

Dendritic cells (DCs) are key antigen presenting cells that have the unique ability to present antigens on MHC molecules, which can lead to either priming or suppression of T cell mediated immune responses. C-type lectin receptors expressed by DCs are involved in antigen uptake and presentation through recognition of carbohydrate structures on antigens. Here we have explored the feasibility of modification of liposomes with glycans for targeting purposes to boost immune responses. The potential of targeting glycoliposomal constructs to the C-type lectin DC-SIGN on DCs was studied using either PEGylated or non-PEGylated liposomes. Our data demonstrate that formulation of the glycoliposomes as PEGylated negatively affected their potential to target to DCs.     

Bioavailability enhancement strategies: basics, formulation approaches and regulatory considerations

Poor solubility remains a major challenge for pharmaceutical industry, which is now considered to be an area of prime importance in the field of biomedical research. Approximately 40% new molecular entities (NMEs) synthesized in pharmaceutical R with advanced combinatorial chemistry and computer aided drug designing (CADD) approaches suffer from poor solubility and bioavailability related issues. Apart from these presence of intestinal tight junctional epithelial cells, transporters and enzymatic barriers further reduces the oral absorption of drugs. Implication of the novel lipid based nanocarriers and nanomaterials like dendrimers and carbon nanotubes as a delivery system can effectively enhance the oral bioavailability of drugs by breaching the barriers, and resolve all critics related to solubility and bioavailability. Thus prime objectives of this review are to give in-depth knowledge and critical appraisal on the barriers for poor oral bioavailability of drugs, along with various novel formulation approaches used for bioavailability enhancement such as lipid based formulations, nanosizing techniques, complexation with polymers and nanomaterials like dendrimers, carbon nanotubes, and penetration enhancers. Also it gives a brief account on in vitro, in vivo screening methods used for assessment of oral bioavailability, and regulatory considerations for the approval.     

Biopharmaceutical characterisation of a low-dose (75 mg) controlled-release aspirin formulation

The release of aspirin from a 75 mg controlled-release formulation, designed to inhibit maximally thromboxane A2 production while sparing stimulated prostacyclin biosynthesis, was characterised in healthy subjects. The calculated in vivo release rate of aspirin matched the design goal of approximately 10 mg h(-1). The C(max) of aspirin associated with the controlled-release formulation was lowered 15-fold relative to a solution formulation of the same dose. The bioavailability of aspirin (based on salicylate concentrations) from the controlled-release formulation was approximately 90% relative to the solution, and drug release was not affected by co-administration of a standard breakfast.     

Triboelectrification of pharmaceutical powders by particle impact

Pharmaceutical powders are very prone to electrostatic charging by colliding and sliding contacts with walls and other particles. In pharmaceutical formulation processes, particle charging is often a nuisance and can cause problems in the manufacture of products, such as affecting powder flow, and reducing fill and dose uniformity. For a fundamental understanding of the powder triboelectrification, it is essential to study charge transfer due to a single contact of a particle with a target plane under well-defined physical, mechanical and electrical conditions. In this study, charge transfer due to a single impact of a particle against a stainless steel target was measured for alpha-lactose monohydrate, aspirin, sugar granules and ethylcellulose. The amount of transferred charge is expressed as a function of impact velocity and impact angle as well as the initial charge. The maximum contact area during impact between a particle and a target plane is estimated by an elastic-plastic deformation model. It is found that the transferred charge is a linear function of the contact area. For a given material, there is an initial particle charge for which no charge transfer occurs due to impact. This is found to be independent of impact velocity and angle, and is hence viewed as a characteristic property, which is related to the contact potential difference and tribo-electric series of the sample powders.     

Inverse gas chromatography: considerations about appropriate use for amorphous and crystalline powders

The use of inverse gas chromatography to assess surface properties of a range of pharmaceutical powders was examined. The powders were two sources of hydroxy propylmethyl cellulose (HPMC), microcrystalline cellulose, magnesium stearate, and acyclovir. These were selected to cover a range for properties from amorphous to crystalline, hydrophilic to hydrophobic, and high to low aqueous solubility. It was found that many powders gave a similar value for the dispersive surface energy, which is surprising given the differences in chemical nature. It is likely that this is due to the use of infinite dilution giving rise to the study of specific regions of the powder surface only. The values obtained for dispersive energies were not influenced by packing mass or flow rate of the carrier gas. The retention of polar probes on the column was a concern for the amorphous HPMC samples. This gave rise to derived values for acid-base nature which varied depending on sample mass and carrier gas flow rate. The data show that care must be taken when studying amorphous samples for which it is possible to obtain diffusion into the material rather than just surface adsorption of probes. Despite these problems, it was still possible to differentiate between the samples (including differences between the two HPMC samples) by use of polar probes. It was also possible to see differences in absorption into the sample, reflecting the different physical forms. For example, microcrystalline cellulose behaved very differently to HPMC. It can be concluded that inverse gas chromatography is a valuable characterization tool, but it must be used with care especially with respect to polar probes on amorphous samples.     

High throughput screening of protein formulation stability: practical considerations.

The formulation of protein drugs is a difficult and time-consuming process, mainly due to the complexity of protein structure and the very specific physical and chemical properties involved. Understanding protein degradation pathways is essential for the success of a biopharmaceutical drug. The present review concerns the application of high throughput screening techniques in protein formulation development. A protein high throughput formulation (HTF) platform is based on the use of microplates. Basically, the HTF platform consists of two parts: (i) sample preparation and (ii) sample analysis. Sample preparation involves automated systems for dispensing the drug and the formulation ingredients in both liquid and powder form. The sample analysis involves specific methods developed for each protein to investigate physical and chemical properties of the formulations in microplates. Examples are presented of the use of protein intrinsic fluorescence for the analysis of protein aqueous properties (e.g., conformation and aggregation). Different techniques suitable for HTF analysis are discussed and some of the issues concerning implementation are presented with reference to the use of microplates.     

Biopharmaceutical characterization of sustained release matrix tablets based on novel carbomer polymers: formulation and in vivo investigation.

With the increased interest in modified release dosage forms and drug delivery systems, there is an increasing concern for biopharmaceutical characterization of the formulation in the early stages of drug product development. The main objectives of biopharmaceutical characterization are the in vitro and in vivo evaluation of the selected formulations in order to identify the factors influencing drug release; define the in vitro test methodology that would be predictive of drug products in vivo behavior and develop quantitative in vitro - in vivo correlation. The purpose of this study was to assess the potential of novel carbomer polymers, Carbopol 971P and Carbopol 71G, as a sustained release agents in matrix tablets containing high dosage drug substance. Although chemically identical, the two polymers exhibited substantially different drug release properties in vitro. Hypothetical in vivo drug release profiles were calculated by numerical deconvolution from cumulative urinary excretion data observed in vivo. The obtained results indicated that sound and reliable in vivo drug release profiles could be obtained from urinary excretion data and also, emphasized the need for in vitro testing under a range of experimental conditions in order to develop the biorelevant drug release methodology.     

Biopharmaceutical considerations of dissolution testing used as a prognostic tool for oral drug adsorption

Drug dissolution is a prerequisite to drug absorption and in vivo effectiveness for almost all drugs given in oral solid dosage forms. Drug absorption depends on the dissolution and solubilization of the drug under physiological conditions, and the permeability across the gastrointestinal tract. Because of the critical nature of the first steps, in vitro dissolution may be relevant to the prediction of biological response. Dissolution tests are applied to choose between formulation factors, assess the lot-to-lot quality of a drug product according to the biobatch; and ensure continuing product quality and performance after certain changes (e.g. in the formulation and manufacturing process, site of manufacture, scale-up). The recently developed Biopharmaceutical Classification System (BCS) has several benefits for recognizing how dissolution tests can be designed and which physiological factors have to be taken into consideration for the in vitro evaluation of solid dosage forms. Choice of test conditions (composition, volume and hydrodynamics of dissolution medium) should be based on where the drug is best absorbed in the gastrointestinal tract, and on whether it is administered in fasted or fed state. Duration of test and physiologically representative media can be selected to simulate gastric and intestinal environments according to the permeability profile of the drug, but mimicking in vivo hydrodynamics remains problematic and further research is required. To provide a basis for predicting the likelihood of achieving a successful in vivo-in vitro correlation, this review summarizes the biopharmaceutical considerations of in vivo drug release in accordance with the BCS.     

Investigation on formulation and preparation of adenovirus encoding human endostatin lyophilized powders

A recombinant adenovirus encoding human endostatin gene, E10A, has finished phase II trials for head and neck cancer. However, the rigid storage temperature (-80°C) and the toxicity of glycerol in the E10A liquid preparation limited its clinical application. In this study, lyophilization was applied to develop a stable E10A lyophilized powder without glycerol that is able to maintain biological activity at 4°C and suitable for intravenous administration. The E10A lyophilized formulations composed of nontoxic and already clinically used excipients were characterized in terms of the pH change during freezing, the eutectic melting temperature (T(eu)) and the collapse temperature (T(c)). Freeze thawing tests were carried out to examine the protective effect of various excipients during freezing. Mannitol and its combinations with sucrose or inulin showed effective protection of E10A. The E10A lyophilized powders were analyzed by particle size measurement, residual humidity quantification, infectivity assay and gene expression level. An optimized formulation (formulation I1) yielded a good recovery of 76% of the starting infectivity after lyophilization and 89% of the original infectivity after storage at 4°C for 180 days. Also the gene expression capability of E10A in formulation I1 was maintained after lyophilization. In addition, it was found that the matrix of amorphous excipients, mannitol combinations with sucrose or inulin, was indispensible in protecting E10A against the stress of freezing and dehydration. Hereby, the E10A lyophilized powder with eliminated glycerol toxicity and improved stability could enhance the applicability of E10A for cancer gene therapy through intravenous administration.     

Key considerations in formulation development for gene therapy products

Gene therapy emerged as an important area of research and led to the success of multiple product approvals in the clinic. The number of clinical trials for this class of therapeutics is expected to grow over the next decade. Gene therapy products are complex and heterogeneous, employ different types of vectors and are susceptible to degradation. The product development process for commercially viable gene-based pharmaceuticals remains challenging. In this review, challenges, stability, and drug product formulation development strategies using viral or non-viral vectors, as well as accelerated regulatory approval pathways for gene therapy products are discussed.