Patent ID: 8389224

Claim:
A method of measuring free drug present in a biological sample from a subject, comprising: (a) combining a biological sample from a subject who is taking the drug with a target cell, wherein the biological sample comprises a biological fluid sample, and wherein the target cell expresses a target that interacts with the drug to bind to at least a portion of the drug present in the biological fluid sample, (b) mixing the target cell of step (a) that has been combined with the free drug present in the biological sample with a labeled anti-Free-Site antibody, wherein the anti-Free Site antibody is a monoclonal antibody that recognizes and binds to an epitope on the target when the drug is not bound to the target but is competitively inhibited by binding of the drug to the target; (c) washing away unbound anti-Free-Site antibody from the target cell of step (b); (d) fixing the target cell of step (c); (e) quantifying the amount of anti-Free-Site antibody binding to the target cell of step (d) by flow cytometry; and (f) determining the amount of free drug present in the biological sample by comparing the amount of anti-Free-Site antibody bound to the target cell as quantified in step (e) to a standard curve of pre-determined amounts of Anti-Free-Site antibody bound to target cells previously exposed to known amounts of the drug, wherein the standard curve illustrates a correlation between the amount of Anti-Free Site antibody bound to target cells and the amount of drug reacted with the target cells, and correlating the amount of anti-Free-Site antibody bound to the target cell to the amount of free drug present in the biological sample based on the standard curve.