Process Patent Protection: Characterizing Synthetic Pathways by Stable-Isotopic Measurements - Pharmaceutical Technology

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Process Patent Protection: Characterizing Synthetic Pathways by Stable-Isotopic Measurements
The authors describe the methods by which precise analyses of stable-isotopic abundances can be used in security and forensic applications for pharmaceutical materials. These methods include product and process authentication of raw materials, pharmaceutical intermediates, drug substances, formulated drug products, and synthetic pathways. Collectively, these methods can be used to investigate and mitigate patent infringement. In the future, more complete examples will be presented containing full isotopic..

Pharmaceutical Technology

Table I: Properties of a four-step synthetic sequence.*
Figure 2 shows illustrative carbon skeletons for reactants and products. Table I summarizes the pertinent quantities. The carbon numbers (n1, n2), initial isotopic compositions (δ1, δ2), fractions of reactants remaining unconsumed (f1, f2), and summed isotope effects (Σε1, Σε2) were chosen to be representative of a typical synthetic scheme. All isotope effects were assumed to be kinetic. Values of δP* (i.e., the isotopic compositions that would be observed if isotopic fractionations were absent) were calculated using Equation 4 with ΔA = ΔB = 0; that is, the simple mass balance Equations 1–2. Values of δP, the isotopic compositions that would actually be observed for the successive products, were calculated using exact forms of integrated rate equations (14).

This example illustrates the interplay of the four factors that control the isotopic compositions of manufactured products, namely the stoichiometries and isotopic compositions of the starting materials, isotope effects associated with the synthetic reactions, and the degree to which conversions of precursors to products are quantitative. The isotopic compositions of all products are dominated by the initial isotopic abundance of the precursor materials and are modulated (viz., depleted) by the degree of completion (f) and the magnitude of any isotopic effects (ε, see Figure 3a). Figure 3b shows a plot that summarizes the difference between the isotopic compositions that are predicted and those that would be observed in the absence of isotope effects (δP* – δP). The last two columns of Table I shows these values. In the first synthetic step, isotope effects on reactant B are rather large, but that reactant is consumed almost completely. The resulting isotopic fractionation is less than 1‰ (the larger value shown in Figure 3b pertains to the product and reflects fractions affecting both reactants).

In the second step, a large isotope effect and poor conversion of reactant C lead to a large isotopic fractionation at the reaction site. Fractionation is diluted, however, now that the product contains 14 carbon atoms. As shown in Figure 3b, the overall difference between real and hypothetical unfractionated products is barely doubled. In the remaining steps, when isotope effects are moderate and the consumption of reactants is relatively efficient, isotopic fractionation declines.

Applications in the pharmaceutical industry

Measuring and tracking isotopic fractionations in synthetic pathways used to prepare pharmaceutical products has potential uses in process analytical chemistry and for the protection of process patents. In process analytical chemistry, the matrix of information obtainable provides a complete isotopic description of pharmaceutical materials from starting materials through synthetic intermediates to final products. Starting materials reacted under consistent conditions with isotopically controlled reagents should always produce products of known isotopic composition. Divergences from the predicted isotopic pathways suggest uncontrolled variables in pharmaceutical manufacture and provide insight into process consistency. When the goal of process analytical chemistry is understanding manufacturing processes, the complete stable-isotopic record of synthesis summarizes many key process variables: reaction rate as affected by the synthetic pathway, reaction rates, temperature, pressure, compound concentration, and so forth.

Observations such as those summarized in Figure 2 can be used to monitor synthetic processes. Once the sequence of isotopic compositions characteristic of a process is known, any variations must be traceable to (1) utilization of reactants from some new source (δ), or (2) variations in the extent to which reactants are converted to products (f), or (3) changes in the reactions used (ε). In this way, very simple and inexpensive analyses integrate information (n, δ, f, ε) of considerable value for understanding the synthetic pathway employed and for process control.


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