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Citric Acid Flavor Masked Zinc Gluconate Lozenge Study
Chapter 4.C.1. - Citric Acid Flavor Masked Zinc Gluconate Lozenge Study

Hard-boiled 4.5 gram sugar and corn syrup lozenges containing 175 mg zinc gluconate (23 mg zinc), 90 mg citric acid as flavor-mask, and lemon flavoring(25) were supplied for testing against placebo in two double-blind clinical trials. One trial was conducted by B. M. Farr and co-workers (32 patients) and the other by J. M. Gwaltney and co-workers (23 patients). No efficacy against HRV-13 and 39 colds was observed.(26) Lozenges had a mild pseudo-astringency. Citric acid was present at 1.33 molar ratio to zinc.(25)

Working in conjunction with the Farr and Gwaltney led studies, Geist and co-workers found little antirhinoviral activity at non-cytotoxic concentrations below 0.03 mMol for either HRV-2 or HRV-39 in vitro -- as determined by cell rounding,(9) in actuality a harmless astringent effect.

Korant reported cell rounding was not an indication of zinc cytotoxicity, and effects noted by Geist and separately by Merluzzi could be induced by many non-cytotoxic agents including a slight change in carbon dioxide concentration and changes to culture medium.(27)

Zinc Ion Availability in the Zinc Gluconate and Citric Acid System

R. Bruce Martin claimed Zn2+ ion concentration to have been 100 percent in saliva -- at pH 2.3, although no evidence to support the salivary pH hypothesis was presented.(25) The hypothesis that pH of saliva produced by an intra-oral dissolution/expectoration of a zinc gluconate with citric acid lozenge would be 2.3 from the acidity of citric acid and lack of buffering by saliva, with consequent 100 percent zinc ion availability(25) can be readily disproved by direct observation. Martin's hypothesis appears to be derived from the observation that the pH of 90 mg citric acid, without zinc or candy lozenge ingredients, in 30 to 35 ml distilled water (roughly equivalent to the amount of saliva generated using such lozenges) is pH 2.3.

NOTE: Upon the retirement of R. Bruce Martin in 1994, he admitted in a leter dated May 9, 1994, that his report was faulty. He wrote, "I very much regret being forced into this theoretical calculation, and have wished for some time that I had not succumbed."

In tests by the present author and others, lozenges chemically identical to those used in this report were subjected to analysis. When dissolved in 30 ml distilled water, the lozenges produced a pH of 3.2. Six unrelated individuals dissolved 12 replicated lozenges and expectorated all saliva flow. Average saliva volume was 35 milliliters. Lozenges required 15 minutes to dissolve. Average pH of 12 intra-oral dissolution/expectorations was 4.35 + 0.08,(28) a value at which little free Zn2+ ions exist, as shown in Figures 11 and 12 below.(25,29) No astringency or salivary protein precipitate in expectorated saliva was noted, further suggesting absence of Zn2+ ions.

Even with five times the amount of citric acid used in the no-effect lozenges in otherwise identical lozenges, a salivary pH of 2.3 was not possible. The augmented formulation produced a pH of 2.9 in saliva but these lozenges caused significant oral pain and were offensively acidic and bitter in taste.

The first stability constant of zinc citrate is log K1 = 4.7 at 37C.(25,29,30-32) At the salivary pH of zinc gluconate-citrate and higher, the amount of Zn2+ ion is considerably less than from zinc gluconate. At physiological pH 7.4 there are no Zn2+ ions, and several negatively charged species predominate (see figures 11 and 12 below).(25,29)

The only physiologically relevant pH is 7.4, as all others are quickly buffered to pH 7.4 in blood, lymph and tissue.(33,34) Negatively charged zinc complexes are repelled from cell surfaces, because cell surfaces are always electronegative,(35) and they have no theoretical or reported antirhinoviral, astringency, interferon-inducing, or cell plasma membrane stabilizing function.

From each perspective (stability constants, and zinc ion availability), the experimental lozenges tested by Farr and co-workers, and Gwaltney and co-workers(26) released no zinc ions at physiologic pH.

Mole fraction zinc with 1.3 mole citric acidFigure 11. Mole fraction Zn2+ species versus pH for solution with Zn2+ and excess citric acid (H3L). At any pH, the curves sum to unity. Curves were constructed from the stability-constant logarithms in parentheses after each reaction: Zn2++HL2-<=>ZnLH (3.0), Zn2++L3-<=>ZnL- (4.8), and ZnL-+L3-<=>ZnL-24- (1.7). Except for the ratio of the last two complexes, the general shapes of the curves are independent of the specific concentrations of 18 mMol Zn2+ and 23 mMol citric acid. Successive pKa values for citric acid deprotonations are pK1=3.0, pK2=4.4, and pK3=5.8. Not included in the analysis is a small amount of ZnLH2+ of low stability likely to form near pH 3. Figure courtesy of B.D. Martin and Antimicrobial Agents and Chemotherapy.(24)

Speciation for zinc citrate Figure 12. Mole fraction for equimolar zinc and citric acid at 10 mMol. 1=Zn2+, 2=ZnL-, 3=ZnLH0, 4=ZnL24-, 5=Zn2L2(OH)2 Figure courtesy of Guy Berthon and JCS-Dalton.(29)

Results of Study

Clinical efficacy of lozenges containing 23 mg zinc from zinc gluconate chelated by extramolar citric acid was assessed in experimentally induced rhinovirus infection in two randomized controlled trials in susceptible adult volunteers.(26) In trial 1, lozenges containing either zinc gluconate (23 mg elemental zinc) chelated with citric acid or lozenges containing a placebo were given 36 hours after nasal inoculation of rhinovirus type 39. Lozenges were administered eight times per day for 5 days. All volunteers had early cold symptoms at the time treatment was begun. In trial 2, the same lozenge regimen was used beginning 2 hours after nasal inoculation with rhinovirus type 13 and was continued for 7 days.

Estimate of effect of ZIA -11 lozenges Figure 13. Estimated effect of ZIA -11 zinc gluconate-citrate and placebo lozenges.

Therapy with zinc gluconate chelated by extra molar citric acid did not reduce severity or duration of cold symptoms or frequency or duration of viral shedding in either trial and appeared to increase severity of common cold symptoms on day 7 (see Figure 13). A breakdown of symptom scores in both studies showed a statistically significant difference in symptom severity with recipients of citric acid-chelated zinc having more severe symptoms than placebo recipients on day 7. Recipients of citric acid-chelated zinc showed trends to higher mucus weight and a greater number of facial tissues used, but neither trend was statistically significant. During the studies, these patients had a 17 percent higher total symptom score, 15 percent higher mucus weight, and used 58 percent more paper tissues than placebo-treated patients.(26)

Considering the day 7 data, zinc gluconate with extramolar citric acid appears to have increased the duration of common colds by approximately 1 day. Placebos also contained citric acid, lemon oil, and a bitter substance, denatonium benzoate. Zinc and placebo lozenges did not have identical tastes, or even similar tastes, but patients had statistically similar perceptions of activity of placebo and zinc lozenges.(36)

Zinc Ion Availability

The results of the clinical trials of Farr and co-workers, and Gwaltney and co-workers(26) do not represent the effect of Zn2+ ions in treatment of common colds. Negatively charged zinc citrate species (pseudo-astringents) in this study increased duration of common colds by approximately 1 day. The best estimate for the ZIA value of the citric acid-chelated zinc gluconate lozenges is negative 11, using linear ZIA relationships (See Chapter 5, Figure 19 equation) to project the value from an estimated 1-day increase in the duration of common colds.

Chapter 4.C.2. - Australian 10 mg Zinc Acetate Effervescent Lozenge Study

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