Aqueous biochemistry

The chemistry of gallium as it pertains to physiologic solutions has been reviewed,9,12 14 and will be only briefly summarized here.

Under physiological conditions, gallium is trivalent in aqueous solution (Ga3+).15 The small and highly charged Ga3+ ion (ionic radius 0.62 A (octahedral) and 0.47 A (tetrahedral)16) is hydrolyzed nearly completely over a wide pH range, forming various hydroxide species, particularly Ga(OH)4~ (gallate). Concomitant with hydroxide formation in aqueous solutions is the formation of H3O+, with resulting lowered pH. If the pH is raised, highly insoluble amorphous Ga(OH)3 precipitates. This Ga(OH)3 tends to convert on aging to the apparently stable crystalline phase GaO(OH).15 Thus, 'free' Ga3+, which is actually tightly coordinated to six water molecules, has low solubility in most aqueous solutions. At pH 7.4 and 25 °C, overall gallium solubility in equilibrium with crystalline GaO(OH) is approximately 1 mM; there is essentially no unbound Ga3+, and 98.4% of the dissolved gallium is present as Ga(OH)4~ and 1.6% as Ga(OH)3.15,17 Both Ga(OH)3 and GaO(OH) display the amphoteric properties first predicted by Mendeleev,1 becoming increasingly soluble with increasing acidity or basicity, though even at pH 2 their solubility is only approximately 10~2M, and at pH 10 it is only approximately 10~3'3 M.15

These factors of gallium solution chemistry have major implications regarding gallium's therapeutic use. When gallium salts, such as the chloride or nitrate, are dissolved in water (or in other aqueous solutions such as normal saline), most of the gallium ions are hydrolyzed as described above, leaving the resulting solution highly acidic. Over time, such solutions, unless they are extremely dilute or are further acidified, are not stable, and some precipitation of gallium hydroxides will occur. Being acidic, the solutions are not appropriate for parenteral administration by injection. To overcome these problems, gallium solutions for injection are usually prepared with citrate, which chelates the gallium, preventing hydrolysis and improving stability.

Further in this regard, it has been repeatedly observed that low gallium absorption occurs when gallium salts are administered orally.4,18,19 This low absorption is likely due in large part to the formation of poorly soluble gallium hydroxides in the gastrointestinal tract.

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