Stabilization by Incorporation into Liposomes Micelles or Emulsions

Entrapment of drug substances in liposomes and micelles can lead to changes in their stability. When entrapment reduces the degradation rate, it can be used as a method for stabilizing pharmaceuticals. Aspirin can be partially stabilized by incorporation in L- a-dimyristoylphosphatidylcholine (DMPC)-based liposomes.489491 Anesthetics such as procaine are also stabilized by incorporation in liposomes.490 Physostigmine salicylate in a phospholipid emulsion is stabilized through interaction with phospholipids at the oil-water interface and through incorporation into the internal phase of the emulsion (Fig. 131).549

mole ratio of HP-p-CD to drug

Figure 129. Stabilization of aspirin against degradation with increasing HP- f -CD concentration at different pH

values at 40°C. A, pH 6.0;O, pH 1.3; • - pH 4.0; A pH 3.0. (Reproduced from Ref. 546 with permission.)

mole ratio of HP-p-CD to drug

Figure 129. Stabilization of aspirin against degradation with increasing HP- f -CD concentration at different pH

values at 40°C. A, pH 6.0;O, pH 1.3; • - pH 4.0; A pH 3.0. (Reproduced from Ref. 546 with permission.)

Figure 130. Effect of y-CD on the degradation of daunorubicin at 50°C, showing stabilization at pH values below 4.5 but accelerated degradation at pH values above 4.5. [Y-CD] = 1.6 x 10-2M;H, no CD. (Reproduced from Ref. 547 with permission.)

Alkaline hydrolysis of forskolin is inhibited in a lipid emulsion.550 Indomethacin in an oil-water gel is solubilized in the surfactant aggregates and stabilized against acid- and base-catalyzed hydrolysis.551

Glycerol, a viscosity-inducing agent, apparently stabilizes drug substances by inhibiting drug diffusion. Photolysis of vitamin is inhibited by addition of glycerol.552

Figure 131. Stabilization of physostigmine salicylate degradation by incorporation into a phospholipid emulsion at pH 5.0. • Solution; ■ phospholipid emulsion. (Reproduced from Ref. 549 with permission.)

Table 9. Effect of Antioxidants on Lovastatin Oxidation"

Antioxidant®

Rate constant (x 10-3 min-1)

Inhibition (%)

Oi-Tocopherol BHAc

Propyl gallate None

53 44

a Reference 553.

c BHA, Butylated hydroxyanisole.

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