Rapamycin: 95% of the drug is sequestered within erythrocytes

I came across the following:

“Because 95% of the drug is sequestered within erythrocytes, whole blood has been recommended as the sample matrix ²”

From a paper titled;

Validation of an assay for routine monitoring of sirolimus using HPLC with mass spectrometric detection.

Copy of this paper at;


Copy and paste the link above if you receive a blank page.

The drug is raperymicin.

The reference they give is;

² Yatscoff RW, Boeckx R, Holt DW, Kahan BD, LeGatt DF, Sehgal S, et al. Consensus guidelines for therapeutic drug monitoring of rapamycin: report of the consensus panel. Ther Drug Monit 1995;17:676-80.

Attached is a copy.

00007691-199512000-00022.pdf (382.5 KB)


The reference paper says “In whole blood, ~95% of RAPA is sequestered within erythrocytes.” That’s potentially a very different statement from the quote from the first paper, as RAPA is probably distributed widely in other sites than blood.

My view based on the paper is that the erythrocytes are carrying/transporting the rapamycin. Not free flowing in the blood.

The reference has to do with the analytical measurement of rapamycin.

“Because 95%of drug is sequestered in red blood cells,[13] the preferred matrix for sirolimus measurement is whole blood”


Whole blood, serum, and various plasmas are not interchangeable sample matrices. While some analytes may give similar results, equivalence can only be ensured by testing matched samples. Whole blood contains RBCs that may occupy as much as 60% of the volume. Some analytes, such as Parathyroid Hormone (PTH), partition freely between the red blood cells and the plasma so that whole blood and plasma values are the same within experimental accuracy. Others, such as lipoproteins, are strongly influenced by red blood cell content. Still others, such as some immunosuppressant drugs like cyclosporin, partition between RBCs and plasma in a temperature-dependent manner that reflects the temperature history of the sample. To avoid this problem, whole blood is the preferred sample for measuring immunosuppressant drugs.