Novel Drug Delivery Systems to Enhance Oral Bioavailability and Their Evaluation

Introduction

Over a past few decades, research efforts in modern pharmaceutical industry have developed new formulations which are capable of enhancing activity of the drug. At the same time novel drug delivery system like nanoparticles, Nanosuspension, etc have been also developed.¹ Lipids have been used as alternative carriers for polymeric nanoparticles, particularly for lipophilic pharmaceuticals and lipid nanoparticles. Which are known as solid lipid nanoparticles (SLNs).

SLNs introduced in 1991 represent alternative and suitable systems to traditional colloidal carriers such as emulsions, liposomes and polymeric micro and nanoparticles. The system consists of spherical solid lipid particles in the nanometre ranges, which are dispersed in water or in aqueous surfactant solution. SLNs are made of solid hydrophobic core having a monolayer of phospholipids coating. The hydrophobic chains of phospholipids incorporated in the fat matrix and have the potential to carry lipophilic or hydrophilic drugs or diagnosticsand arrival of pharmacologically dynamic specialists so as to accomplish the site-explicit activity of the medication at the remedially ideal rate and portion routine. Despite the fact that liposomes have been utilized as potential transporters with extraordinary focal points including protection of drugs from debasement, focusing to site of activity and decrease poisonous quality or reactions,their applications are constrained because of natural issues, for example, low exemplification proficiency, quick spillage of water-dissolvable medication within the sight of blood parts and poor storage stability. Then again, polymeric nanoparticles offer some favourable circumstances over liposomes.²

Solid lipid nanoparticles are one of the novel potential colloidal carrier systems as alternative materials to polymers which is identical to oil in water emulsion for parenteral nutrition, but the liquid lipid of the emulsion has been replaced by a solid lipid.Theyhave many advantages such as good biocompatibility, low toxicity, and lipophilic drugs are better delivered by solid lipid nanoparticles and the system is physically stable.³

Oral administration is the most usually and generally utilized course for drug transport because of its accommodation and is for the most part favoured by the patient. It offers numerous points of interest among the different drug transport courses, for example, effortless and high tolerant consistence, the GI-tract go about as an obstacle for the few bioactive molecules. To beat the issues related with the oral organization, the nanocarrier frameworks are used. The in vivo presentation and bioavailability of any medication relies upon the in vitro rate and degree of disintegration of the measurements structure. In this manner, a few progresses have been created for the upgrade of solvency, disintegration rate, and in vivo bioavailability of medications, with the goal that medication mixes made reasonable for the advancement of ideal plan. The epic methods for the improvement of dissolvability and bioavailability of medication applicants are nano emulsions, strong lipid nanoparticles, strong scatterings, and polymeric nanoparticles.⁴

Diabetes mellitus (DM) has been known to humanity for over 2000 years. DM is a continual and metabolic issue coming about because of absence of insulin discharge, an overall absence of insulin activity, or body cells are impervious to insulin described by high glucose levels. Level of diabetes event is expanding step by step because of way of life and age. One of the choices of using nanomedicines in the administration of diabetes is ending up most encouraging decision. Diabetes influenced 380 million individuals worldwide. SLN’s are gaining more consideration due to its low poisonous quality, high physical security, great biodegradability, biodegradation, and their capacity to convey both hydrophilic and hydrophobic medications financially viable and supported transports.5

The term bioavailability, one of the head pharmacokinetic properties of drug, is utilized to reproduce the division of a managed portion of unaltered drug that arrives at the fundamental course. When a drug is directed intravenously, its bioavailability is 100%. In any case, when a medicine is regulated through different courses, (for example, oral), its bioavailability diminishes (because of inadequate ingestion or first-pass digestion). Figure 1 reproduces various pathways of drug ingestion from gastrointestinal tract to foundational dissemination. The estimation of the measure of the medication in the plasma at intermittent time interims in a roundabout way demonstrates the rate and degree at which the dynamic pharmaceutical fixing is assimilated from the medication item and ends up accessible at the site of activity. Bioavailability is one of the basic apparatuses in pharmacokinetics, as it must be viewed as when ascertaining measurements for non-intravenous courses of organization.6

Linagliptin is an oral antidiabetic drug used in the treatment of type-2 diabetes mellitus which acts by inhibiting the enzyme dipeptidyl peptidase-4 (DPP-4). It belongs to BCS class-III drug i.e., it has high solubility and low permeability and has a low bioavailability of 30%. This article shows how different techniques are used to improve oral bioavailability of linagliptin and one among techniques is SLNs.⁷

Advantages of solid lipid nanoparticle preparation⁸

•  Nanoparticles can be controlled by different courses like oral, nasal, parenteral, etc.
• SLNsupgrade the fluid dissolvability of ineffectively solvent medication and improve the bioavailability of drug.
• Improved bioavailability of poor water-soluble molecules.
• Useful to analyse different diseases.
• Use of biodegradable physiological lipids decreaseacute and chronic toxicity.
• As focused on medication transporter, nanoparticles lessen poisonous quality and upgrade proficient of drug.
• Change the technique for medication conveyance to improve client acknowledgment or diminish assembling cost.
• Possibility of controlled drug release and drug targeting.

Factors influencing bioavailability⁹

The supreme bioavailability of a drug, when regulated by an extravascular course, is generally short of one (for example F<1). Different physiological variables diminish the accessibility of drug preceding their entrance into the systemic circulation, they are: -

•  Physicochemical properties of the drug (hydrophobicity, pKa, solubility)
• The drug formulation (immediate release, excipients used, manufacturing methods, modified release - delayed release, extended release, sustained release, etc.)
• Whether the drug is administered in a fed or fasted state
• Gastric emptying rate
• Transporters: Substrate of an efflux transporter (e.g. P-glycoprotein)
• Interactions with other drugs (e.g. antacids, alcohol, nicotine)
• Interactions with foods (e.g. grapefruit juice, cranberry juice)

Reasons for poor bioavailability¹⁰

Poor aqueous solubility

The substance of gastrointestinal tract is fluid and hence a drug having poor water solubility has a low immersion solvency which is regularly corresponded with a low disintegration speed, bringing about poor oral bioavailability.

Inappropriate partition coefficient

Too hydrophilic drug would not have the option to pervade through the gastrointestinal mucosa and too lipophilic medication won’t break down in the watery gastrointestinal substance. For ideal retention, the drug should have adequate fluid solvency to break down in the gastrointestinal substance and furthermore satisfactory lipid solubility to encourage its division into the lipoidal film and after that into systemic circulation.

First-pass metabolism

Orally controlled drug must go through the intestinal wall and after that through the entry flow to the liver; both are common side of first pass (digestion of a drug before it arrives at systemic circulation). In this manner, numerous medications might be processed before sufficient plasma concentration which are about to showpoor bioavailability. The enterocyte communicates many of the metabolic chemicals that are communicated in the liver. These incorporate cytochromes P450, UDP-glucanosyltransferases, sulfotransferases, and esterase’s. The weakness of a medication to initially pass digestion by CYP3A4 impacts the oral bioavailability, which diminishes as the degree of first pass metabolism increases.

Degradation in the gastrointestinal tract

Drug substances utilized as pharmaceuticals have different atomic structures and may be, in this way, inclined to numerous and variable degradation pathways. Protein drugs, specifically are profoundly powerless to inactivation because of the pH and the chemicals present in gastrointestinal tract.

Degradation due to low pH in stomach

Most drug substances are genuinely steady at the neutral pH found in the small digestive tract (ignoring enzymatic degradation) yet can be unstable at low pH values found in the stomach. Instances of drug that are extremely corrosive labile are different penicillin’s, erythromycin and some of its analogues, and the 2’, 3’- dideoxy purine nucleoside enemies of AIDS drugs. Learning of the steadiness of a drug in the pH scope of 1-2 at 37°C is significant in the definition structure of conceivably corrosive labile drug. At low pH, foscarnet decays through a corrosive catalysed decarboxylation; subsequently, poor oral bioavailability (7-9%) may be because of deterioration of foscarnet in gastric corrosive.

Interaction with food

Drug that experiences a critical first-pass metabolism with a lower bioavailability running from 5 to 30% might be influenced to a more prominent degree by grapefruit juice. Calcium, just as nourishment and dairy items containing high concentrations of calcium, may diminish the absorption of antibiotic drug because of chelate development in the gut.

Techniques for improving bioavailability¹¹

• Nanosuspensions
• Microemulsion, self-micro emulsifying drug delivery system
• Solid lipid nanoparticles, polymeric nanoparticles
• Vesicular delivery systems such as liposomes, noisome, etc.

Nanosuspension

Nanosuspensions are colloidal scattering and biphasic framework comprising of drug particles scattered in a fluid medium in which the measurement of the suspended particles is under 1μm in size. Nanosuspension can be created by a proper size-decrease strategy and balanced out by an appropriate stabilizer. As per Noyes–Whitney and Ostwald–Freundlich standards, the molecule size in the nanometre range can prompt expanded disintegration speed and immersion solvency for a nanosuspension, which is generally joined by an expansion in bioavailability. Nanosuspensions can be set up by two techniques, in particular, “base up innovation” and “top down innovation”. Base up innovation is a technique to frame nanoparticles like precipitation, microemulsion, and liquefy emulsification strategies. Top down innovation includes the breaking down of bigger particles into nanoparticles, instances of which are high-weight homogenization and processing techniques. In base up innovation, the nanoprecipitation strategy displays various points of interest, just like a direct method and being quick and simple to perform. The medication is disintegrated in a dissolvable, which is then added to nonsolvent.

Microemulsion, Self-micro emulsifying drug delivery system

To increase the water solubility and disintegration rate of ineffectively solvent drug to acquire quicker beginning of activity, limit the fluctuation in retention, and improve its general oral bioavailability. There has been expanding centre around the utility of self-small scale emulsifying drug transporter (SMEDDS) as these lipid-based details are accounted for to help the absorption of water less soluble drug by diminishing the inherent limitation of moderate and fragmented disintegration and by encouraging the arrangement of a microemulsion inside the digestive system that is fit for keeping up generally a poorly soluble drug in solution. Notwithstanding every one of these methodologies, readiness of lipid-based detailing was attempted to make plan process simpler. The primary point of the examination was to grow ineffectively solvent drug SMEDDS to enhance the solubility of the drug which will make them bear on the bioavailability. The SMEDDS comprises of an isotropic blend of drug, lipid, surfactant, and normally a cosurfactant or cosolvent. At the point when presented to the fluids of the gastrointestinal (GI) tract, these precursor solutions precipitously emulsify to frame profoundly scattered microemulsions. These scatterings regularly display molecule measures beneath 300 nm (SEDDS) or 100 nm (SMEDDS) and have been appeared to increase the oral bioavailability of lipophilic medications.

Solid lipid nanoparticles, polymeric nanoparticles

Liquid formulation can give high break down drug levels and progressively fast discharge, yet can present more noteworthy difficulties as for low drug loading and physical and compound dependability contrasted and strong structures. Nanocrystals can possibly build bioavailability of disintegration constrained drugs by expanding the surface region, and subsequently the disintegration rate, of low-solvency drug crystal. In circumstances where expanding the disintegration rate isn’t adequate to satisfactorily build bioavailability, undefined drug shapes that can give break down drug levels higher than crystalline solubility can be beneficial. The medication/polymer nanoparticles portrayed here are correlative to other bioavailability-upgrading innovations and have great qualities that may position them to empower for applications where fast disintegration or potentially expanded break down drug levels are required. By and large, strong shapeless scatterings can give high bioavailability to low-solvency drugs. Furthermore, these nanoparticles, as different undefined scatterings, can give disintegrated tranquilize focuses higher than crystalline solubility, which add to quicker absorption rates and higher all out absorption.

Vesicular delivery systems such as liposomes, niosomes, etc.

Vesicular transport framework has indicated focal points over regular dose shapes in oral drug transport. Notwithstanding increased solvency and disintegration rates, these bearers give an amazing way to maintain a strategic distance from first-pass metabolism through incitement of lymphatic vehicle, prompting improved bioavailability. As of late, spheroid vesicular structures formed by self-assembly of nonionic surfactants (Noisome), have gotten critical consideration as drug transport frameworks. Although niosomes are like liposomes in physical properties and biopharmaceutical capacities, they have focal points, including higher security, simpler taking care of and capacity, and lower cost, making them a promising option. These characteristics likewise make them promising devices in the oral delivery of different therapeutic agent. By changing the physicochemical qualities of nanocarriers, including molecule size, flexibility, and surface charge, it might be conceivable to structure vesicles with characteristics proper for a given application. It has been accounted for that the organic destiny of vesicular frameworks following oral organization is influenced by the consideration of bile salts and charge-instigating Agents.

Evaluation and characterization

In vitro diffusion studies:

In-vitro drug dissemination of nanoparticles is completed by dialysis bag dispersion strategy. A 4–5 cm long bit of the dialysis tubing was made into a dialysis sac by collapsing and tying up one part of the tubing with string. It was then topped off with phosphate-supported and inspected for the breaks. The sac was then discharged and 1 mL of the nanosuspension, fluid nanosuspension was precisely moved into sacs, which filled in as the donor compartments. The sacs were by and by inspected for release and after that suspended in the glass measuring glasses containing 50 mL phosphatebuffer, which become the receptor compartment. At predetermined time interval, 3 mL tests were pulled back from the receptor compartment and investigated spectrophotometrically. New buffer was utilized to renew the receptor compartment at each time point.¹²

Drug entrapment efficiency¹²

Separation of free drug: Analysis of linagliptin from SLN was done by separating free drug from the nanoparticle’s dispersion. The separation was done by centrifugation of nanoparticles. Then, the nanoparticles pellets and supernatant were separated out.

Direct method:

In this method, analysis of drug from SLN was done by dissolving in phosphate buffer pH 7.4. The dispersion was then allowed to stand for overnight for complete dissolution of drug. Then, absorbance was taken against phosphate buffer as a blank on UV-Visible Spectrophotometer. The percentage entrapment was calculated by using following equation. %Drug Entrapment =Drug entrapped in SLN’s/ Total drug taken ×100 Indirect method: In this method, analysis of drug from SLNs was done by appropriately diluting supernatant in phosphate buffer and absorbance was taken against phosphate buffer as a blank on UV-Visible Spectrophotometer. To find out percentage entrapment following equation was used. [8-9?] %Drug entrapment = Total drug taken - drug in supernatant/Total drug taken ×100.

Drug loading

Drug loading of the developed SLNs were determined as the amount of drug loaded (as a percentage) in relation to the solid phase and drug. The % drug loading was calculated by the following formula:

Drug loading= Total amount of drug-amount of free drug/Total weight of the nanoparticle×100% 

Stability study and determination of shelf life

The stability investigation of the SLNs was completed according to rules given in the ICH Q1A (R2) (ICH subject Q1 (R2), 2009). The example was put away at various temperatures with moistness for a time of 6months in steadiness chamber. The examples were pulled back at predetermined time interval (0, 30, 60, 90, and 180d) and examined for any adjustment in the molecule size, sedate stacking and embodiment productivity. Further, the diagram was plotted between log% medication remaining versus time (in days) to decide the degradation rate constant (K) where, slant of the bend was resolved from the graph.¹³

Slope= - k/2.303

where K is the degradation rate constant. The shelf life of SLNs was calculated at 25°C by calculating the time required to degrade 10% of the drug in the NP from the following equation.

t10%=0.1054/k 25° C

where, t10% is the time required to degrade 10% of the drug in the NPs.

Rheological studies

Rheological properties (investigation of deformation and flow of matter) are required in different pharmaceutical area. It screens the impact of vehicles consistency on arrival of drug from the arrangements and consequent percutaneous retention. Likewise, it is significant from the assembling perspective. Thickness estimations were completed utilizing a Brookfield viscometer (T – bar shaft). The plan of SLN was kept in 100mL beaker and dial readings was noted at 3, 5, 6, 10, 12, 20, 30, 50 and 60 rpm. The speed was then progressively brought down and the comparing dial readings were noted.¹⁴

Drug content

The drug identical to 25 mg of formulation was taken and dissolved in little amount of methanol. At that point the formulation is warmed on the water shower so the drug present in the detailing was totally dissolved. At that point the arrangement was filtered through Whattman filter paper in 25 mL volumetric flask and volumes were made sufficient by methanol to give concentration of 1000 μg/mL for clotrimazole. At that point 1 mL was pipetted out in 100 mL volumetric flask to give concentration of 10μg/mL and absorbance was estimated at 261nm.15 In-vitro release studies In Franz dispersion cell, 2 gm of test was kept in donor compartment. The whole surface of cellophane membrane was in contact with the receptor compartment containing 62 mL of phosphate buffer pH 7.4. The receptor compartment was ceaselessly mixed utilizing the magnetic stirrer. The temperature was kept at 35°C. The study was completed for 24 hrs and the example was pulled back at 30-minute time interval and same volume was replaced with free phosphate buffer solution. The substance of clotrimazole from pulled back example was estimated after suitable dilution.¹⁶ Stability studies Whenever new formulation is developed, it is exceptionally fundamental to set up that the therapeutic activity of drug has not experienced any change. To accommodate this, the chosen formulations were exposed to stability studies. For the most part, the perception of the rate at which the product degrade under typical room temperature requires long time. To avoid undesirable delay, the standards of the quickened accelerated studies are adopted. The International Conference of Harmonization rules titled “strength testing for drug substance and product” describe the stability tests necessities for drug enlistment applications in the European Union, Japan, and United States of America. Short term Stability studies were completed at 40 ± 2°C/75 ± 5%% RH for the selected formulation for physical and chemical stability for 1month.15 Determination of percentage yield The percentage production yield was calculated from the weight of dried nanoparticles recovered from each of the formulations and the sum of initial dry weight of starting

%Yield= weight of nanoparticles/ weight of drug +polymer × 100

Percentage yield: The obtained solid lipid nanoparticles were weighed. Percentage yield was calculated by using following formula.

% yield =practical yield/ theoretical yieldx 100

Determination of PH

The pH was determined by utilizing computerized pH meter. Fifty mL of formulation was taken in a beaker. At that point, the bulb of the pH meter was dipped into the formulation and the pH was estimated accurately.¹⁷

Measurement of particle size, polydispersity index and zeta potential

Molecule size dispersion of linagliptin loaded Solid lipid nanoparticles T-SLN and linagliptin were determined by laser scanning procedure by using nanoparticle SZ-100 (Horiba Company, Japan) instrument after appropriate dilution with double distilled water. The mean molecule size, polydispersity index, and zeta potential were determined for T-SLN and linagliptin and polydispersity list used to quantify the size circulation population of nanoparticles.¹⁷

Particle size determination

The estimation of zeta potential considers expectation about the storage stability of colloidal particles, as the molecule collection will be less to the charged particles. For the readied SLNs, the zeta potential (mV), molecule size and polydispersity index.¹⁷

CONCLUSION

Solid lipid nanoparticles of linagliptin may show promising effect to improve or enhance the oral bioavailability of drug. Solid lipid nanoparticle shows better therapeutic action and italso reduces dose frequency and toxicity. The enhancement of oral bioavailability of linagliptin can be achieved by using different techniques like Solid lipid nanoparticle, microemulsion, nanosuspension and vesicular drug delivery systems like liposomes, and niosomes.

ABBREVATIONS SLN:

Solid Lipid Nanoparticle GIT: Gastro Intestinal Tract SMEDDS: Self Micro Emulsifying Drug Delivery System CYP3A4: Cytochrome P450 3A4