Oral formulation strategies to improve solubility of poorly water-soluble drugs

INTRODUCTION: In the past two decades, there has been a spiraling increase in the complexity and specificity of drug-receptor targets. It is possible to design drugs for these diverse targets with advances in combinatorial chemistry and high throughput screening. Unfortunately, but not entirely unexpectedly, these advances have been accompanied by an increase in the structural complexity and a decrease in the solubility of the active pharmaceutical ingredient. Therefore, the importance of formulation strategies to improve the solubility of poorly water-soluble drugs is inevitable, thus making it crucial to understand and explore the recent trends. AREAS COVERED: Drug delivery systems (DDS), such as solid dispersions, soluble complexes, self-emulsifying drug delivery systems (SEDDS), nanocrystals and mesoporous inorganic carriers, are discussed briefly in this review, along with examples of marketed products. This article provides the reader with a concise overview of currently relevant formulation strategies and proposes anticipated future trends. EXPERT OPINION: Today, the pharmaceutical industry has at its disposal a series of reliable and scalable formulation strategies for poorly soluble drugs. However, due to a lack of understanding of the basic physical chemistry behind these strategies, formulation development is still driven by trial and error.     

Refining stability and dissolution rate of amorphous drug formulations

Introduction: Poor aqueous solubility of active pharmaceutical ingredients (APIs) is one of the main challenges in the development of new small molecular drugs. Additionally, the proportion of poorly soluble drugs among new chemical entities is increasing. The transfer of a crystalline drug to its amorphous counterpart is often seen as a potential solution to increase the solubility. However, amorphous systems are physically unstable. Therefore, pharmaceutical formulations scientists need to find ways to stabilise amorphous forms.

Areas covered: The use of polymer-based solid dispersions is the most established technique for the stabilisation of amorphous forms, and this review will initially focus on new developments in this field. Additionally, newly discovered formulation approaches will be investigated, including approaches based on the physical restriction of crystallisation and crystal growth and on the interaction of APIs with small molecular compounds rather than polymers. Finally, in situ formation of an amorphous form might be an option to avoid storage problems altogether.

Expert opinion: The diversity of poorly soluble APIs formulated in an amorphous drug delivery system will require different approaches for their stabilisation. Thus, increased focus on emerging techniques can be expected and a rational approach to decide the correct formulation is needed.     

Potentials and challenges in self-nanoemulsifying drug delivery systems

Introduction: A significant number of new chemical entities (almost 40%), that are outcome of contemporary drug discovery programs, have a potential therapeutic promise for patient, as they are highly potent but poorly water soluble resulting in reduced oral bioavailability. Self-nanoemulsifying drug delivery systems (SNEDDS) have emerged as a vital strategy to formulate these poorly soluble compounds for bioavailability enhancement.

Areas covered: The review gives an insight about potential of SNEDDS with regards to oral drug delivery. The effect of various key constituents on formulation of SNEDDS and their applications in oral drug delivery is also discussed. Various aspects of formulation, characterization and biopharmaceutical aspects of SNEDDS are also been explored. The choice and selection of excipients for development of SNEDDS is also discussed.

Expert opinion: The ability of SNEDDS to present the drug in single unit dosage form either as soft or hard gelatin capsule with enhanced solubility maintaining the uniformity of dose is unique. With the ease of large-scale production, high drug-loading capacity, improvement in release behavior of poorly water-soluble drugs and improvement of oral bioavailability, SNEDDS have emerged as preferable system for the formulation of drug compounds with bioavailability problems due to poor aqueous solubility.     

Insoluble drug delivery strategies: review of recent advances and business prospects

The emerging trends in the combinatorial chemistry and drug design have led to the development of drug candidates with greater lipophilicity, high molecular weight and poor water solubility. Majority of the failures in new drug development have been attributed to poor water solubility of the drug. Issues associated with poor solubility can lead to low bioavailability resulting in suboptimal drug delivery. About 40% of drugs with market approval and nearly 90% of molecules in the discovery pipeline are poorly water-soluble. With the advent of various insoluble drug delivery technologies, the challenge to formulate poorly water soluble drugs could be achieved. Numerous drugs associated with poor solubility and low bioavailabilities have been formulated into successful drug products. Several marketed drugs were reformulated to improve efficacy, safety and patient compliance. In order to gain marketing exclusivity and patent protection for such products, revitalization of poorly soluble drugs using insoluble drug delivery technologies have been successfully adopted by many pharmaceutical companies. This review covers the recent advances in the field of insoluble drug delivery and business prospects.KEY WORDS: Bioavailability, Cocrystals, Solubility, Inclusion complexation, Nanoparticles, Self-emulsifying formulations, Proliposomes     

Recent advances in drug delivery strategies for treatment of ovarian cancer

Introduction: Ovarian cancer is associated with the highest mortality rate of all gynecological malignancies, due in part to inadequate treatment strategies and the asymptomatic nature of the disease. Current standard of care includes surgery and systemic chemotherapy. However, this approach can result in toxicities and eventual disease relapse, due to the emergence of multidrug resistance. Drug delivery systems (DDS) have shown promise in overcoming many of the limitations facing conventional treatment regimens.

Areas covered: This review provides an overview of recent advances in DDS strategies for the treatment ovarian cancers. Nano-sized systems, including nanoparticles, micelles, liposomes and drug conjugates; microspheres; implants and injectable depots are discussed. The advantages, limitations and clinical potential of these strategies are also outlined.

Expert opinion: Nano-sized DDS enable passive targeting to tumors due to their size, and further improvements in tumor localization can be made using targeting moieties. Microspheres, implants and injectable depots have been investigated for peritoneal localized and sustained therapy. Overall, the benefits of using DDS for ovarian cancer therapy include higher drug levels at the diseased site, circumvention of drug resistance mechanisms, minimization of non-specific toxicities, improvements in solubility of poorly soluble drugs and elimination of toxicities associated with conventionally used pharmaceutical excipients.     

Drug release from liquisolid systems: speed it up,slow it down

Introduction: Today, the properties of many new chemical entities have shifted towards higher molecular weights and this in turn increases the lipophilicity hence decreasing aqueous solubility. The low solubility of drugs usually has in vivo consequences such as low bioavailability, increased chance of food effect and incomplete release from the dosage form.

Areas covered: The present review discusses the advantages of the liquisolid technology in formulation design of poorly water soluble drugs for dissolution enhancement and highly water soluble drugs for slow release pattern.

Expert opinion: With the advent of high throughput screening and combinatorial chemistry, it has been shown that most of the new chemical entities have a high lipophilicity and poor aqueous solubility, hence poor bioavailability. In order to improve the bioavailability, the release rate of these drugs should be enhanced. Although there are multiple technologies to tackle this issue, they are not cost effective due to the involvement of sophisticated machinery, advanced preparation techniques and complicated technology. As the liquisolid technology uses a similar production process as the conventional tablets, this technology to improve the release rate of poorly water soluble drugs will be cost effective. This technology also has the capability to slow down drug release and allows preparing sustained release tablets with zero order drug release pattern. The excipients required for this technology are conventional and commonly available in the market. The technology is in the early stages of its development with extensive research currently focused on. It is envisaged that the liquisolid compacts could play a major role in the next generation of tablets.     

Drug nanosuspensions: a ZIP tool between traditional and innovative pharmaceutical formulations

Introduction: A nanosuspension or nanocrystal suspension is a versatile formulation combining conventional and innovative features. It comprises 100% pure drug nanoparticles with sizes in the nano-scale range, generally stabilized by surfactants or polymers. Nanosuspensions are usually obtained in liquid media with bottom-up and top-down methods or by their combination. They have been designed to enhance the solubility, the dissolution rate and the bioavailability of drugs via various administration routes. Due to their small sizes, nanosuspensions can be also considered a drug delivery nanotechnology for the preparation of nanomedicine products.

Areas covered: This review focuses on the state of the art of the nanocrystal-based formulation. It describes theory characteristics, design parameters, preparation methods, stability issues, as well as specific in vivo applications. Innovative strategies proposed to obtain nanomedicine formulation using nanocrystals are also reported.

Expert opinion: Many drug nanodelivery systems have been developed to increase the bioavailability of drugs and to decrease adverse side effects, but few can be industrially manufactured. Nanocrystals can close this gap by combining traditional and innovative drug formulations. Indeed, they can be used in many pharmaceutical dosage forms as such, or developed as new nano-scaled products. Engineered surface nanocrystals have recently been proposed as a dual strategy for stability enhancement and targeting delivery of nanocrystals.     

Drug delivery strategies for poorly water-soluble drugs: the industrial perspective

Introduction: For poorly soluble compounds, a good bioavailability is typically needed to assess the therapeutic index and the suitability of the compound for technical development. In industry, the selection of the delivery technology is not only driven by technical targets, but also by constraints, such as production costs, time required for development and the intellectual property situation.

Areas covered: This review covers current developments in parenteral and oral delivery technologies and products for poorly water-soluble compounds, such as nano-suspensions, solid dispersions and liposomes. In addition, the use of biorelevant dissolution media to assess dissolution and solubility properties is described. Suggestions are also included to systematically address development hurdles typical of poorly water-soluble compounds intended for parenteral or oral administration.

Expert opinion: A holistic assessment is recommended to select the appropriate delivery technology by taking into account technical as well as intellectual property considerations. Therefore, first and foremost, a comprehensive physico-chemical characterization of poorly water-soluble compounds can provide the key for a successful selection and development outcome. In this context, the identified physical form of the compound in the formulation is used as a guide for a risk–benefit assessment of the selected oral delivery technology. The potential of nano-suspensions for intravenous administration is unclear. In the case of oral administration, nano-suspensions are mainly used to improve the oral absorption characteristics of micronized formulations. The development of an in situ instantaneous solubilization method, based on stable, standardized liposomes with low toxicity, opens new avenues to solubilize poorly water-soluble compounds.     

Solid dispersions,Part II: new strategies in manufacturing methods for dissolution rate enhancement of poorly water-soluble drugs

Introduction: The absorption of poorly water-soluble drugs, when presented in the crystalline state to the gastrointestinal tract, is typically dissolution rate-limited, and according to BCS these drugs belong mainly to class II. Both dissolution kinetics and solubility are particle size dependent. Nowadays, various techniques are available to the pharmaceutical industry for dissolution rate enhancement of such drugs. Among such techniques, nanosuspensions and drug formulation in solid dispersions are those with the highest interest.

Areas covered: This review discusses strategies undertaken over the last 10 years, which have been applied for the dissolution enhancement of poorly water-soluble drugs; such processes include melt mixing, electrospinning, microwave irradiation and the use of inorganic nanoparticles.

Expert opinion: Many problems in this field still need to be solved, mainly the use of toxic solvents, and for this reason the use of innovative new procedures and materials will increase over the coming years. Melt mixing remains extremely promising for the preparation of SDs and will probably become the most used method in the future for the preparation of solid drug dispersions.     

Self-emulsifying drug delivery system and the applications in herbal drugs

Abstract

Herbal drugs have been used for thousands of years in the east and have had a recent resurgence in popularity among consumers in the west. However, most of herbal drug are poorly soluble and have hydrophobic properties and poor distribution, leading to reduced bioavailability and hence decreased treatment efficacy, requiring repeated administration or increased dose. In the past few decades, considerable attention has been focused on the development of self-emulsifying drug delivery system (SEDDS) for herbal drugs. SEDDS is isotropic and thermodynamically stable solutions consisting of oil, surfactant, co-surfactant and drug that can spontaneously form oil-in-water micro/nanoemulsion when mixed with water under gentle stirring. The formulation can be a viable alternative to classical formulations to take advantage of their lipophilic nature and to solve their problems of poor solubility, poor bioavailability, low oral absorption and instability. The mechanism of self-emulsification, solubility studies, construction of phase diagram, optimization and characterization of herbal drugs-loaded SEDDS formulation and in situ absorption evaluation of herbal drugs in rat intestine are presented in our article.     

Ionic liquids in drug delivery

Introduction: To overcome potential problems with solid-state APIs, such as polymorphism, solubility and bioavailability, pure liquid salt (ionic liquid) forms of active pharmaceutical ingredients (API-ILs) are considered here as a design strategy.

Areas covered: After a critical review of the current literature, the recent development of the API-ILs strategy is presented, with a particular focus on the liquefaction of drugs. A variety of IL tools for control over the liquid salt state of matter are discussed including choice of counterion to produce an IL from a given API; the concept of oligomeric ions that enables liquefaction of solid ILs by changing the stoichiometry or complexity of the ions; formation of ‘liquid co-crystals' where hydrogen bonding is the driving force in the liquefaction of a neutral acid–base complex; combining an IL strategy with the prodrug strategy to improve the delivery of solid APIs; using ILs as delivery agents via trapping a drug in a micelle and finally ILs designed with tunable hydrophilic-lipophilic balance that matches the structural requirements needed to solubilize poorly water-soluble APIs.

Expert opinion: The authors believe that API-IL approaches may save failed lead candidates, extend the patent life of current APIs, lead to new delivery options or even new pharmaceutical action. They encourage the pharmaceutical industry to invest more research into the API-IL platform as it could lead to fast-tracked approval based on similarities to the APIs already approved.     

Lipid Microemulsions for Improving Drug Dissolution and Oral Absorption: Physical and Biopharmaceutical Aspects

Purpose. This review highlights the state-of-the-art in pharmaceutical microemulsions with emphasis on self-emulsifying systems, from both a physical and biopharmaceutical perspective. Although these systems have several pharmaceutical applications, this review is primarily focused on their potential for oral drug delivery and intestinal absorption improvement. Methods. Physicochemical characteristics and formulation design based on drug solubility and membrane permeability are discussed. Results. Case studies in which lipid microemulsions have successfully been used to improve drug solubilization/dissolution and/or intestinal absorption of poorly absorbed drugs/peptides are presented. Conclusions. Drug development issues such as commercial viability, mechanisms involved, range of applicability, safety, scale-up and manufacture are outlined, and future research and development efforts to address these issues are discussed.     

Cyclodextrin and phospholipid complexation in solubility and dissolution enhancement: a critical and meta-analysis

Introduction: Poor solubility and dissolution of drugs are the major challenges in drug formulation and delivery. In order to improve the solubility and dissolution profile of drugs, various methods have been investigated so far. The cyclodextrin (CD) complexation and phospholipid (PL) complexation are among the exhaustively investigated methods employed for more precise improvement of the solubility and dissolution of poorly water-soluble drugs.

Areas covered: The article discusses the CD and PL complexation techniques of solubility and dissolution enhancement. Various studies reporting the CD and PL complexation as the potential approaches to improve the dissolution, absorption and the bioavailability of the drugs have been discussed. The article critically reviews the physicochemical properties of CDs and PLs, eligibility of drugs for both the complexation, thermodynamics of complexation, methods of preparation, characterization, advantages, limitation and the meta-analysis of some studies for both the techniques.

Expert opinion: The CD and PL complexation techniques are very useful in improving solubility and dissolution (and hence the bioavailability) of biopharmaceutical classification system Class II and Class IV drugs. The selection of a particular kind of complexation can be made on the basis of eligibility criteria (of drugs) for the individual techniques, cost, stability and effectiveness of the complexes.     

Potential of polymeric nanoparticles in AIDS treatment and prevention

Importance of the field: Acquired immunodeficiency syndrome (AIDS) remains one of the greatest challenges in public health. The AIDS virus is now responsible for > 2.5 million new infections worldwide each year. Despite significant advances in understanding the mechanism of viral infection and identifying effective treatment approaches, the search for optimum treatment strategies for AIDS remains a major challenge. Recent advances in the field of drug delivery have provided evidence that engineered nanosystems may contribute to the enhancement of current antiretroviral therapy.

Areas covered in this review: This review describes the potential of polymeric nanoparticle-based drug delivery systems in the future treatment of AIDS. Polymeric nanoparticles have been developed to improve physicochemical drug characteristics (by increasing drug solubility and stability), to achieve sustained drug release profile, to provide targeting to the cellular and anatomic human immunodeficiency virus (HIV) latent reservoirs and to be applied as an adjuvant in anti-HIV vaccine formulations.

What the reader will gain: The insight that will be gained is knowledge about the progress in the development of polymeric nanoparticle-based drug delivery systems for antiretroviral drugs as alternative for AIDS treatment and prevention.

Take home message: The advances in the field of targeted drug delivery can result in more efficient strategies for AIDS treatment and prevention.     

Overcoming formulation challenges for the next generation of vaccines

Introduction: In recent years, the number of active pharmaceutical ingredients with high therapeutic impact, but very low water solubility, has increased significantly. Thus, a great challenge for pharmaceutical technology is to create new formulations and efficient drug-delivery systems to overcome these dissolution problems.

Areas covered: Drug formulation in solid dispersions (SDs) is one of the most commonly used techniques for the dissolution rate enhancement of poorly water-soluble drugs. Generally, SDs can be defined as a dispersion of active ingredients in molecular, amorphous and/or microcrystalline forms into an inert carrier. This review covers literature which states that the dissolution enhancement of SDs is based on the fact that drugs in the nanoscale range, or in amorphous phase, dissolve faster and to a greater extent than micronized drug particles. This is in accordance to the Noyes–Whitney equation, while the wetting properties of the used polymer may also play an important role.

Expert opinion: The main factors why SD-based pharmaceutical products on the market are steadily increasing over the last few years are: the recent progress in various methods used for the preparation of SDs, the effect of evolved interactions in physical state of the drug and formulation stability during storage, the characterization of the physical state of the drug and the mechanism of dissolution rate enhancement.     

pH modulation: a mechanism to obtain pH-independent drug release

Importance of the field: In formulation development, weakly acidic or basic drugs pose a major challenge as the solubility depends significantly on pH of the dissolution media. This gives rise to pH-dependent drug release, as the formulation is exposed to different pH ranges in the gastrointestinal tract. This indicates a need to carry out formulation optimization for such drugs while developing them into a dosage form.

Areas covered in this review: For overcoming pH-dependent behavior of drugs, pH-modifying excipients (which alter the microenvironment pH inside the formulation) are most commonly used. A combination of enteric and sustained release polymers can be used for weakly basic drugs. Other strategies include conversion of crystalline drug to amorphous form, enhancement of partitioning of unionized fraction of drug from the formulation, and using a combination of pH modifier and enteric polymer, micellar solubilization and inclusion complexation.

What the reader will gain: Readers will gain an insight into various formulation techniques for obtaining pH-independent drug release for weakly acidic and basic drugs.

Take home message: Readers will be able to evaluate the different formulation strategies in terms of their applicability and best use of the available strategies when designing their own research work for such drugs.     

New drug-targeting strategy from beneath the shell of egg

Importance of the field: Antibiotic resistance is a serious problem that continues to challenge the healthcare sectors and has become increasingly alarming in the past few years. To face this emerging global crisis, there is a need to find a new class of antibiotics that act on new microbial targets and/or harness existing antibiotics by developing new drug-targeting strategies.

Areas covered in this review: This review: explores an innovative drug-delivery strategy of using hen egg lysozyme as a carrier to enable water solubilization and to allow specific targeting to the microbial cells of a water-insoluble antimicrobial agent with a powerful killing action; addresses potentials for lysozyme in antibiotics drug targeting; and provides insight for the future direction of this highly prospective technology.

What the reader will gain: The unique features and advantages of lysozyme-based drug delivery system are highlighted. The efficiency of lysozyme in solubilization and delivery of lipophilic antibiotics, to reformulate drugs that may fail clinical trials owing to low solubility, is emphasized.

Take home message: Fewer pharmaceutical companies are inventing new antibiotics because of long development times and high failure rates. Combining lysozyme with a powerful old antibiotic may open doors to revolutionizing medicine, particularly in the treatment of deadly infections.     

From nanoemulsions to self-nanoemulsions,with recent advances in self-nanoemulsifying drug delivery systems (SNEDDS)

Introduction: Lipid-based drug delivery systems (LBDDS) are the most promising technique to formulate the poorly water soluble drugs. Nanotechnology strongly influences the therapeutic performance of hydrophobic drugs and has become an essential approach in drug delivery research. Self-nanoemulsifying drug delivery systems (SNEDDS) are a vital strategy that combines benefits of LBDDS and nanotechnology. SNEDDS are now preferred to improve the formulation of drugs with poor aqueous solubility.

Areas covered: The review in its first part shortly describes the LBDDS, nanoemulsions and clarifies the ambiguity between nanoemulsions and microemulsions. In the second part, the review discusses SNEDDS and elaborates on the current developments and modifications in this area without discussing their associated preparation techniques and excipient properties.

Expert opinion: SNEDDS have exhibit the potential to increase the bioavailability of poorly water soluble drugs. The stability of SNEDDS is further increased by solidification. Controlled release and supersaturation can be achieved, and are associated with increased patient compliance and improved drug loads, respectively. Presence of biodegradable ingredients and ease of large-scale manufacturing combined with a lot of ‘drug-targeting opportunities’ give SNEDDS a clear distinction and prominence over other solubility enhancement techniques.     

Inorganic matrices: an answer to low drug solubility problem

Introduction: Many active pharmaceutical ingredients (APIs), in development and already on the market, show a limited and variable bioavailability mainly associated to inadequate biopharmaceutical properties such as aqueous solubility and dissolution rate. The latter is the main factor responsible for the limited, and sometimes inadequate, efficacy of many orally administered drugs, belonging to class II and IV of the Biopharmaceutics Classification System (BCS). Moreover, because of their low solubility, such drugs require high doses to be administered in order to obtain their pharmacological effect, increasing the side effect incidence.

Areas covered: The present review reports the most common technological approaches intended to improve solubility and dissolution rate of BCS class II and IV drugs such as nanocrystals, solid dispersions, cyclodextrins and solid lipid nanoparticles. Particular attention will be focused on the use of inorganic matrices (lamellar anionic clays and mesoporous materials) as host for the delivery of poor soluble APIs (guest).

Expert opinion: The employment of inorganic matrices for the realization of host–guest composites is a suitable strategy for the biopharmaceutical properties enhancement. This objective can be achieved without any modification of API chemical structure.     

Complexity in the therapeutic delivery of RNAi medicines: an analytical challenge

Introduction: Nucleic acids have witnessed a dramatic acceleration in their therapeutic exploitation and currently represent a growing number of applications in drug development pipelines. However, a more wide-spread development of therapeutics based on nucleic acids is restricted by their poor chemical and enzymatic stability in vivo, lack of cellular uptake and insufficient capability to reach intracellular targets.

Areas covered: Advanced formulation of nucleic acids in nano-sized carriers may help unlocking their potential as therapeutic agents. Nano-sized matters own specific features responsible for inducing characteristic interactions with biological molecules and tissues. These properties enable for the enhancement of the nano-formulation’s therapeutic efficacy, but on the other hand, the nanomatters interactions in biological fluids are also responsible for adverse effects. The purpose of this review is to reflect on the complexity in the therapeutic delivery of RNA interference-based drugs emerging from the recent clinical experiences and report the actual technological and analytical advances introduced to solve it.

Expert opinion: The complexity in the therapeutic delivery of nucleic acids and the heterogeneity of side effects make the interpretation of the therapeutic outcome difficult. Hence the development of analytical approaches applicable in the field of nucleic acid delivery is becoming a major challenge.