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Technical Note: The authors investigated the variables important for calcium-alginate formation as well as dissolution.
A previous study provided supporting data for the concept that anti-human immunodeficiency virus (HIV) drugs, as represented by aurintricarboxylic acid (ATA), can be released from a calcium–alginate complex through the action of citrate derived from seminal fluid (1). This approach is especially attractive because the highest concentration of the anti-HIV drug would be released only during insemination and would, theoretically, be most effective for protecting women against infection from HIV-positive sexual partners. While the intended route of application is intravaginal, it is possible that intrarectal administration could potentially provide chemoprophylaxis for men and women.
This approach may be a useful adjunct to condoms and anti-HIV drugs in gels. The initial study provided dissolution data for a single concentration of citrate and aurintricarboxylic acid in a range of calcium–alginate formulations, where both calcium chloride and sodium alginate were used in equivalent concentrations (from 0.5–2.0% w/v). The purpose of the present study was to investigate the effect of variables (i.e., calcium and alginate) in the formulation and to investigate citrate as the most important physiological factor for dissolution. Although ATA was used in the previous study, the focus on calcium and alginate as variables is more important when considering other anti-HIV drugs (2–5).
Calcium–alginate gels may be formed readily into various sizes and shapes from dry, ultrathin films that rapidly release drug upon simple hydration (i.e., immediately after intravaginal and intrarectal application) to suppositories with sustained-release characteristics (6). The present study also investigates the ability of cellulose to enhance viscosity in the formulation without interfering with the ionic interactions between calcium, alginate, and citrate.
Calcium chloride, sodium citrate, microgranular α-cellulose, and aurintricarboxylic acid (sodium salt) were obtained from Sigma-Aldrich (St. Louis, MO). Sodium alginate as Protanal LF 200 was obtained from Protan (Portsmouth, NH). The first formulation trial examined the effect of citrate on dissolution. In this trial, two different formulations were evaluated. At a fixed concentration of ATA in alginate, where each mL of alginate (i.e., 0.5 and 2.0% w/v) contained 1 mg of ATA, the ATA was immobilized in calcium alginate with equal (w/v) concentrations of calcium chloride and sodium alginate. Beads of calcium–alginate ATA complex (~25 μL/bead) were produced by dropwise addition of the ATA–alginate mixture into aqueous solutions of calcium chloride. The beads were stored in water at 4 °C until tested. For citrate dissolution testing, 1 mL of sodium citrate (66 or 132 mM at pH 8.0) was added to five calcium–alginate ATA beads placed in each well of a 24-well Corning Costar 3473 microtiter plate (Corning, Corning, NY) and incubated at 23 °C.
The time necessary for total dissolution of the beads was gauged by visual inspection. The microtiter plates were mixed by an MS-3 digital mixer (IKA, Staufen, Germany) at 500 rpm. Procedures for the second formulation trial (for testing the effect of calcium in the formulation) were identical, except that beads were produced by adding 0.5 and 2.0% alginate solutions to 0.5, 1.0, and 2.0% (w/v) calcium-chloride solutions. A single concentration of citrate was used for dissolution testing (i.e., 132 mM). For fabrication of suppositories without ATA, 1 g of cellulose was thoroughly mixed with 20 mL of 2% (w/v) alginate solution to prepare a cellulose–alginate suspension. The suspension (2 mL) was loaded into a 10-cc syringe (No. 305482, BD, Franklin Lakes, NJ) from which the end was removed at the 1-mL index mark, yielding an open bore. The suspension was extruded from the syringe directly into 100 mL of 2% (w/v) calcium-chloride solution (at 23 °C) and stirred by oscillation for 20 min in an orbital mixer (Belly Button, Stovall, Greensboro, NC).
The suppositories contained approximately 2 mL each and were stored in distilled water at 4 °C until tested. For dissolution testing, each suppository was placed into one well of a six-well, Corning Costar 3471 microtiter plate (Corning) and incubated at 23 °C. Each well contained 5 mL of sodium citrate at a concentration of 132 mM. The microtiter plates were mixed with an MS-3 digital mixer at 250 rpm. The time necessary for total dissolution of the suppositories was gauged by visual inspection.
Results and conclusion
Within the range of calcium and alginate tested, alginate was confirmed as the most important formulation variable in this system and was associated with the greatest change in dissolution time (see Figures 1 and 2). Variation in calcium yielded little change in the dissolution time (see Figure 2).
Figure 1: Dissolution in citrate of 0.5 and 2.0% calcium-alginate gels containing aurintricarboxylic acid (1 mg/mL). (FIGURES ARE COURTESY OF T.J. SMITH)
Although a single concentration of ATA was used, variations in the concentration of this polyanion must be considered if ATA and its derivatives are to be advanced for preclinical or clinical evaluation. Nevertheless, these results reflect the importance of alginate-concentration changes, regardless of the anti-HIV agent under consideration. The results also verified the importance of citrate concentration because dissolution time appeared to be inversely proportional to citrate concentration (see Figure 1). Citrate in seminal fluid at a concentration of ~132 mM) is reduced by as much as 50% in certain disease processes (7). While this reduction in citrate would be expected to decrease the release rate of drug from the formulation, total dissolution occurred at both citrate concentrations. Microgranular cellulose yielded suppositories of a prototype that may yield sustained-release characteristics in the presence of citrate. The properties of the suppositories and the dissolution time are summarized in Table I.
Figure 2: Effect of calcium chloride on the citrate-mediated dissolution of 0.5 and 2.0% alginate gels. The citrate concentration was 132 mM.
In addition to its utility to viscosity enhancement for manual fabrication of the prototype suppositories, cellulose is relatively inexpensive and safe to use for intravaginal and intrarectal applications (4, 5). This formulation's capability to alter release rates for anti-HIV drugs, as well as the simplicity of design, deserves further consideration and investigation.
Table I: Characteristics of manually fabricated prototype suppositories.
Timothy J. Smith* is a professor and chair of the Department of Physiology and Pharmacology, and Lara Haddad, Soraia Faizyar, Evelyn Valdez, Elizabeth Gramer, Layla S. Derreja, and Stefany M. Bowers are doctoral students, all at Thomas J. Long School of Pharmacy, University of the Pacific, Stockton, CA 95211, tel. 209.946.3168, fax 209.946.2857, firstname.lastname@example.org.
*To whom all correspondence should be addressed.
Submitted: Aug. 25, 2009. Accepted: Oct. 7, 2009.
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