Pharmaceutical solids absorption into

Modern infrared (IR) spectroscopy is a versatile tool applied to the qualitative and quantitative determination of molecular species of all types. Its applications fall into three categories based on the spectral regions considered. Mid-IR (MIR) is by far the most widely used, with absorption, reflection, and emission spectra being employed for both qualitative and quantitative analysis. The NIR region is particularly used for routine quantitative determinations in complex samples, which is of interest in agriculture, food and feed, and, more recently, pharmaceutical industries. Determinations are usually based on diffuse reflectance measurements of untreated solid or liquid samples or, in some cases, on transmittance studies. Far-IR (FIR) is used primarily for absorption measurements of inorganic and metal-organic samples. 

The BCS is a scientific framework for classifying active pharmaceutical ingredients based upon their aqueous solubility and intestinal permeabihty. When combined with the dissolution of the pharmaceutical product, the BCS takes into account three major factors that govern the rate and extent of drug absorption (exposure) from immediate-release oral solid dosage forms dissolution, solubility, and intestinal permeability. 

Combining the dissolution of the pharmaceutical product with these two properties of the API, takes the three major factors that govern the rate and extent of drug absorption from immediate-release solid dosage forms into account (27). On the basis of their dissolution properties, immediate-release dosage forms can be categorized as having very rapid , rapid , or not rapid dissolution characteristics. 

Dissolution is simply a process by which a solid substance goes into solution. The determination of dissolution rates of pharmaceutical substances from dosage forms does not predict their bioavailability or their in vivo performance rather, it indicates the potential availability of drug substance for absorption. Therefore, it is essential for pharmacists and pharmaceutical scientists to know and understand the importance of dissolution and its potential influence on the rate and extent of absorption and availability for drugs. 

Active substances that are dissolved in water are, if they remain in solution, immediately available for absorption after taking on an empty stomach. If they precipitate in the acidic environment of the empty stomach the precipitation will often be so fine that it easily passes the closed pylorus and re-dissolves, after which rapid absorption is still possible. The pharmaceutical availability (actually the dissolution rate of the solid substance) of suspensions for oral use depends on many factors including solubility in vivo, the crystal modification and the particle size and the viscosity of the suspension. In practice it occurs that an active substance (such as phenobarbital) may be formulated as a suspension in water or as a solution in a lipophilic solvent such as acetem (see Sect. 23.3.6). Beware that changing from a suspension into a solution may cause a great increase in absorption rate and thus cause a change in pharmacodynamics. 

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