A Non-Traditional Internal Standard

Background
A large Pharmaceutical Company was developing a new topical formulation from an existing compound. The very low dose level required that a highly sensitive LC/MS/MS assay be developed to monitor systemic exposure.

The Challenge
A stable labeled internal standard was not commercially available, and the Client did not have time to prepare an isotopic internal standard which was crucial for the accuracy and precision of the assay.

The Solution
Our scientists recognized the reactivity of the amino groups on the molecule and proposed the creation of an isotopic internal standard by derivatization with propyl-d7 chloroformate. In the new assay, the drug molecule was derivatized in the plasma with unlabeled propyl chloroformate during the preparation procedure. Both derivatives (d0 and d7) were monitored as drug and internal standard in the mass spectrometer, respectively. This approach increased the sensitivity and robustness of the assay and allowed the Client to maintain their rigorous timeline without resorting to an expensive synthetic effort.