In vitro method for detecting precipitation of parenteral formulations after injection.

Many injectable formulations currently on the market, including diazepam and alprazolam, utilize one or more cosolvents to solubilize the active constituents. On injections into an aqueous medium, some of these components tend to precipitate. A simple procedure is described for measuring the degree of precipitation that occurs when a solubilized drug is injected. This in vitro technique was used to show that alprazolam injection shows less precipitation than diazepam injection under all tested conditions, and that the precipitation observed with diazepam can be controlled by ensuring that the formulation is injected very slowly. This simple technique also can be used during preformulation development to evaluate the relative potential for precipitation of various formulations.

Solubility and partitioning VI: octanol solubility and octanol-water partition coefficients.

A simple equation for the estimation of the aqueous solubility of crystalline solutes was previously derived based on the assumption that the presence of water does not significantly alter the crystal properties of the solute. The data presented verify the solubility equation for a set of 36 nonelectrolytes and weak electrolytes. Using the same set of solutes, the two major assumptions used to derive the equation were also verified: that the octanol solubility of nonelectrolytes is exponentially proportional to the melting point of the solute and that the octanol-water solubility ratio is a good approximation of the octanol-water partition coefficient.

Solubility and partitioning IV: Aqueous solubility and octanol-water partition coefficients of liquid nonelectrolytes.

The aqueous solubility and octanol-water partition coefficient of over 100 nonelectrolyte organic liquid solutes are related by the simple equation log Sw = -1.016 log PC +0.515, where Sw is the molar solubility of liquid solutes in water and PC is the experimental partition coefficient of the solutes in the octanol-water system. The liquids studied represent a wide variety of organic compounds including aliphatic and aromatic hydrocarbons, alcohols, esters, ethers, aldehydes, and ketones. This finding is in agreement with that reported by Hansch and coworkers. However, these results are significant because only the experimental values for the aqueous solubilities and octanol-water partition coefficients are included, as opposed to the calculated partition coefficients used by Hansch. This relationship is extremely useful in understanding the overall solubility and partitioning phenomenon for organic liquids are provides a basis for studying crystalline solids and gases.

Solubility and partitioning I: Solubility of nonelectrolytes in water.

On the basis of a semiempirical analysis, an equation was obtained that enables the estimation of the aqueous solubility of either liquid or crystalline organic nonelectrolytes: (see formula in text) where log PC and delta Sf are estimated from the chemical structure and MP is either known or experimentally determined. Analysis of this equation provides a means of assessing the role of crystal structure [as reflected by the melting point (MP) and the entropy of fusion (delta Sf)] and of the activity coefficient [as reflected by the octanol-water partition coefficient (PC)] in controlling the aqueous solubility of a drug. Techniques are also provided for estimating the entropy of fusion of organic compounds.

Solubility of nonelectrolytes in polar solvents IV: nonpolar drugs in mixed solvents.

The molecular and group surface area approach to solubility is shown to be applicable to mixed aqueous solvent systems. An equation is derived which is consistent with the exponential increase in the aqueous solubility of nonpolar drugs that frequently accompanies the addition of a cosolvent. This equation predicts that: (a) the ability of a drug to be solubilized by a cosolvent is proportional to its hydrophobic surface area per molecule, and (b) the ability of a cosolvent to solubilize any drug is inversely proportional to its interfacial tension against a reference liquid hydrocarbon. These predictions are experimentally verified with solubility studies of several alkyl p-aminobenzoates in propylene glycol-water mixtures and of hexyl p-aminobenzoate in mixtures of water the ethanol, methanol, ethylene glycol, propylene glycol, glycerin, and formamide.

Partition coefficients and surface areas of some alkylbenzenes.

The experimentally measured log P values (logarithms of partition coefficients) of a number of alkylbenzenes are shown to be quantitatively related to the hydrocarbon surface area HSA of the molecule by pi = 0.0275 X HSA - 0.863 (correlation coefficient =0.996, standard deviation = 0.071). The use of surface area as a correlating parameter eliminates the need for correction factors to account for branching, cyclization, ring fusion, and "backfolding". Futhermore, surface area calculations provide a conceptual basis for understanding how conformation can effect partitioning.