Injectable Formulations of Poorly Water-Soluble Drugs

Abstract

A growing number of new therapeutic molecules are limited by low or erratic bioavailability due to poor water solubility. Because of the clinical demand for new and more efficacious anti-cancer, antiviral, and anti-infective drugs, many of these new drugs must be formulated for injection. Poor water solubility can be addressed by a range of formulation approaches such as pH manipulation, salt formation, and cosolvent and surfactant addition, or by more advanced techniques such as complexation, liposomal encapsulation, or nanosuspension. While remaining focused on drug solubility, issues such as buffering, tonicity, sterility, and drug ­product stability also must be considered when formulating injectable products. This chapter outlines a formulator’s approach toward development of an injectable drug product containing an active ingredient with poor solubility. Marketed injectable products, listings of GRAS excipients, and techniques for enhancing solubility are offered as case studies to assist in the formulation process.

Appendices

Method Capsule 1Solubilization of a Poorly Water-Soluble Drug Using a Surfactant with pH Adjustment

Based on the method reported by Li et al. (1999a).

Objective

• To increase the solubility of flavopiridol, an antineoplastic agent with a water solubility of 0.025 mg/mL, by changing the solution pH and incorporating a surfactant.

Equipment and Reagents

• Flavopiridol (apparent pK _a 5.86).

• Polysorbate 80 (PS80).

• Citrate buffer.

• End-over-end mechanical rotator operating at 20 rpm.

• Pinnacle octylamine HPLC column (150 cm x 4.6 mm).

• Mobile phase of 0.1% triethylamine in 50 mM phosphate buffer pH 2.5 and acetonitrile in a ration of 35:65 (flow rate 1 mL/min; detection 263 nm).

• Syringe filter unit (0.45 μm).

• pH meter.

Method

• Prepare solutions of varying concentrations of PS80 (0, 2.5, 5, 10, and 20%) in citrate buffer with a pH of 4.3 and 8.4.

• Perform solubility study by placing 0.5 mL of each solution into vials with excess flavopiridol.

• Attach vials to rotator, place at 25°C, and rotate at 20 rpm for 6 days or until equilibrium solubility is achieved. (Note: The drug is stable for >2 months under these conditions).

• Filter samples through 0.45-μm syringe filter membrane and check final solution pH.

• Perform HPLC analysis of samples for potency (solubility) of flavopiridol.

Results

• The solubility of flavopiridol in solution without surfactant was 1.37 mM and 0.055 mM at pH 4.3 and 8.4, respectively.

• The solubility of a drug in the micelle is influenced by the micellar partition coefficient and drug water solubility.

• Increasing the concentration of PS80 resulted in a substantial linear increase in drug solubility. The increase was considerably greater (up to ∼27 mM at 20% PS80) at pH 4.3, where the majority of drug is ionized.

• Solubilization of the ionized drug by PS80 is more critical than that of the unionized species for increasing the overall solubility due to the increased water solubility resulting from the increased solubility of drug in the micelles.

Method Capsule 2Solubilization of a PWS Drug using a Combination of Cosolvent and Surfactant

Based on the method reported by Dong et al. (2008).

Objective

• To develop a stable injectable solution of N-epoxymethyl-1,8-naphthalimide, a nonionizable PWS drug (0.0116 mg/mL) that is very susceptible to hydrolytic degradation by screening a number of solubilization techniques.

Equipment and Reagents

• N-epoxymethyl-1,8-naphthalimide (ENA).

• Cosolvents: ethanol, polyethylene glycol 400 (PEG-400), PG, and glycerol.

• Surfactants: Cremophor EL and Tween 80.

• Complexation agents: HPβCD and hydroxypropyl-γ-cyclodextrin (HPγCD).

Method

• Determine the solubility of ENA in cosolvent–water solutions with concentrations up to 50% (w/v).

• Determine the solubility of ENA in surfactant–water solutions with concentrations up to 20% (w/v).

• Determine the solubility of ENA in aqueous solutions containing up to 20% (w/v) cyclodextrins.

• Based on the results from the above solubility studies, determine the solubility of ENA in nonaqueous solution containing different ratios of the best performing solubilizers: Cremophor EL and ethanol (0:100, 50:50, 60:40, 70:30, and 100:0% v/v).

• Select a number of ratios of Cremophor EL:ethanol from the previous stability study and assess formulation stability upon dilution with saline.

• Assess formulation physical and chemical stability at different storage conditions.

Results

• Aqueous solutions containing concentration of 50% ethanol, PEG400, PG, or glycerol have an ENA solubility of 0.531, 0.269, 0.181, and 0.038 mg/mL, respectively.

• The solubility of ENA is increased to 0.395 and 0.410 mg/mL with 20% of Tween 80 and Cremophor EL, respectively.

• The solubility of ENA is increased to 0.169 and 0.190 mg/mL with 20% of HPβCD and HPγCD, respectively.

• Cremophor EL:ethanol ratios of 0:100, 50:50, 60:40, 70:30, and 100:0 increased the solubility of ENA to 1.25, 3.90, 4.54, 5.42, and 6.65, respectively.

• Of all the ratios screened, only a combination of 70% Cremophor EL and 30% ethanol containing 4 mg/mL of ENA did not precipitate upon dilution (5- to 20-fold) with saline. This formulation is physically and chemically stable for over 4 months at either 5°C or room temperature.

Method Capsule 3Solubilization Using a Combination of Complexation Agent and Cosolvent

Based on the method reported by Li et al. (1999b).

Objective

• To increase the solubility of fluasterone, a nonpolar oncology compound, using various amounts of cosolvent (ethanol) in combination with the complexation agent hydroxyl propyl-β-cyclodextrin (HPβCD).

Equipment and Reagents

• Fluasterone (intrinsic solubility 0.045 μg/mL)

• Ethanol

• HPβCD

• End-over-end mechanical rotator operating at 20 rpm

• Pinnacle octylamine HPLC column (150 cm x 4.6 mm)

• Mobile phase of 75% acetonitrile in water (flow rate 1.1 mL/min; detection 220 nm)

• Syringe filter unit (0.45 μm)

• pH meter

Method

• Prepare solutions of varying concentrations of ethanol (0–70%) and HPβCD (0–20%).

• Place 0.5 mL of each solution into glass vials with excess flavopiridol.

• Connect vials to rotator, place at 25°C, and rotate at 20 rpm for 6 days or until equilibrium solubility is achieved. (Note: The drug is stable for >50 days under these conditions).

• Filter samples through a 0.45-μm syringe filter membrane and check final solution pH.

• Perform HPLC analysis of samples for potency of fluasterone.

Results

• An exponential increase in the solubility of fluasterone is observed in a cosolvent and water system with increasing ethanol concentration.

• Using HPβCD in an aqueous solution yields a linear increase in drug solubility corresponding with increasing HPβCD concentration.

• As ethanol concentration increases from 0.2 to 25.06% in the presence of HPβCD, the polarity of the solvent decreases. The result is reduced drug complexation within the HPβCD cavity and a decrease in the drug solubility over this range, with a minimum at 25.06%.

• At high ethanol concentrations (25.06–75%), the solubility increases due to the increase in the concentration of free drug and ternary complex.

• With the ethanol concentration held constant (>25.06%), there is a linear increase in drug solubility with increasing amounts of HPβCD.

Method Capsule 4Preparation of a Submicronized Injectable Emulsion Formulation

Based on the method reported by Levy and Benita (1989).

Objective

• To design and characterize an emulsion formulation for diazepam, which meets the requirements for an IV or IM administration.

Equipment and Reagents

• Diazepam

• Purified soybean oil

• Egg yolk phospholipids (purified fractionated complex emulsifier)

• Poloxamer (nonionic emulsifier)

• Glycerin (osmotic agent)

• Methyl and butyl p-hydroxybenzoic acid (preservative)

• α-Tocopherol (antioxidant)

• Sodium hydroxide solution (10%)

• High-shear mixer (Polytron)

• Two-stage pressure homogenizer (Gaulin homogenizer)

Method

• The formulation is composed of 0.5% (w/w) diazepam, 20% oil, 1.2% phospholipids, 2% poloxamer, 2.25% glycerin, 0.02% α-tocopherol, 0.2% methyl and 0.075% butyl p-hydroxybenzoic acid, and water for injection to 100 g.

• Perform preparation and processing with nitrogen atmosphere and sterile conditions.

• Add poloxamer, glycerin, and preservatives to the aqueous phase and dissolve.

• Dissolve phospholipids and diazepam in the oil phase containing α-tocopherol.

• Filter both phases, and heat separately to 70°C while mixing with a magnetic stirrer. Increase the temperature to 85°C for the emulsification step using the high-shear mixer.

• Once the emulsion is formed, cool rapidly. A reduced droplet size is achieved using the two-stage Gaulin homogenizer. Adjust pH to the desired level with sodium hydroxide and filter the emulsion.

Results

• Characterization of the formulation including the oil phase properties, pH, phase/volume ratio, emulsifier, phospholipid, nonionic emulsifier, and diazepam concentrations produced an optimized stable emulsion.

• Droplet size was reduced to an acceptable size for IV administration from a mean size of 0.65 μm by a high-shear mixer to 0.27 μm, using the Gaulin two-stage pressure homogenizer.

Method Capsule 5Preparation of Multivesicular Liposome Formulation for Sustained IM Delivery

Based on the method reported by Zhong et al. (2005).

Objective

• To design a multivesicular liposome (MVL) sustained delivery formulation for IM injection.

Equipment and Reagents

• Breviscapine (bioactive ingredient)

• Phosphatidycholine

• Phosphatidylglycerol

• Triolein or tricaprylin

• Cholesterol

• Sucrose and glucose

• Buffers: 50 mM arginine buffer pH 7, 40 mM L-Lysine

• Chloroform–diethyl ether (1:1 v/v)

• T 18 basic Ultra-Turrax mixer

Method

• Use the double-emulsion process to produce breviscapine MVLs. Prepare a lipid mixture in 1 mL chloroform–diethyl ether with 40 mg phosphatidycholine, 8 mg phosphatidylglycerol, 40 mg cholesterol, and triolein or ­tricaprylin at a 5.75:1 molar ratio of phosphatidycholine to total triglyceride content.

• Prepare an aqueous solution containing 40 mg/mL of breviscapine, 4% w/v sucrose in 50 mM arginine buffer pH 7.

• Emulsify the lipid and aqueous solutions to make a W/O emulsion at 10,000 rpm for 8 min with the mixer.

• Prepare the second aqueous solution with 40 mM L-Lysine and 3.4% glucose and emulsify with the W/O emulsion to form a W/O/W emulsion. Transfer to an Erlenmeyer flask and remove the chloroform–diethyl ether by nitrogen flushing the liquid surface at 30°C.

• Centrifuge at 600g for 5 min to remove free breviscapine from the MVLs and resuspend in a buffered saline solution.

Results

• Brevicapine MVLs produced by this method significantly prolonged the release of drug both in vitro (5–6 days) and in vivo (IM injection in rats lasted 4–5 days) compared to other the liposome preparation techniques investigated.

Method Capsule 6Preparation of Nanoemulsions for IV Delivery

Based on the method reported by Kelmann et al. (2007).

Objective

• To prepare a nanoemulsion for IV delivery of a PWS drug, carbamazepine.

Equipment and Reagents

• Carbamazepine (CBZ)

• Ethanol

• Acetone

• Soybean lecithin (lipophilic emulsifier)

• Medium chain triglycerides (MCT)

• Polyoxyl 35 castor oil (lipophilic emulsifier)

• Polysorbate 80 (hydrophilic emulsifier)

• Glycerol (tonicity agent)

• Magnetic stirrer

Method

• Mix CBZ (final emulsion concentration of 2 mg/mL) with castor oil or a 1:1 mixture of castor oil:MCT (w/w) and prepare the lipophilic emulsifier ­(soybean lecithin or polyoxyl 35 castor oil at 6% w/w) by dissolving it in a 50:50 mixture of acetone:ethanol (v/v).

• Make the oil phase by adding the prepared emulsifier to the drug:oil dispersion.

• Prepare the aqueous phase by dissolving 4% (w/w) PS80 and 2.5% (w/v) glycerol in water.

• While stirring (moderate speed) with a magnetic stirrer, slowly add the oil phase to the aqueous phase to form the nanoemulsion.

• Remove solvent and water by reducing the pressure, adjust pH to 7.0 with 0.1 M sodium hydroxide, and store the emulsion at 5°C.

Results

• The emulsion droplets are spherically shaped, have an amorphous core, and range in size from 100 to 250 nm.

• The resulting optimized nanoemulsion formulation demonstrates 3 months ­stability based on droplet size, polydispersity, zeta potential, and drug content data.

• The drug nanoemulsion meets requirements for IV administration and will be tested for in vivo evaluation.

Method Capsule 7Preparation of an IV Nanosuspension Formulation for Reduced Irritation and Phlebitis upon Injection

Based on the method reported by Xiong et al. (2007).

Objective

• To improve upon a current clinical IV formulation for nimodipine, which contains a high concentration of ethanol that causes injection site pain, irritation, and phlebitis.

Equipment and Reagents

• Nimodipine (compound for the treatment of subarachnoid hemorrhage-related vasospasm)

• Sodium deoxycholate

• Poloxamer 188

• Mannitol

• Polysorbate 80

• MC One (fluid jet mill)

• Niro-Soavi NS1001L (high-pressure homogenizer)

• Laboratory freeze drier

Method

• Perform all the following steps under reduced lighting conditions to protect the light-sensitive drug.

• Jet mill the coarse powder of nimodipine to produce a microparticulate powder.

• Disperse with a magnetic stirrer 0.5% (w/v) the milled powder in an aqueous solution composed of 0.6% (w/v) poloxamer 188, 0.4% (w/v) sodium cholic acid, and 4.0% (w/v) mannitol.

• Perform pre-milling using the high-pressure homogenizer (maintain sample temperature at 25–30°C) by starting with the settings at 200 bar with two cycles then increasing to 500 bar with five cycles.

• Follow the pre-milling step with 15–20 cycles at 1,500 bar to produce the nanosuspension.

• Lyophilize the nanosuspension by drying for 15 hr at −15°C (below 200 mTorr), with a secondary step of 3 hr at −5°C, and a final step of 2 hr at 20°C.

• Sterilize by gamma irradiation for 6 hr with an absorbed dose of 12 kGy.

Results

• Injection toleration studies performed in rabbits demonstrate that this formulation reduces occurrence of phlebitis and minimizes local irritation when compared with the clinical ethanol-based product.