Calcitriol: After 0.5 µg Cmax <2× basal level [Bioanalytics]

posted by Helmut Homepage – Vienna, Austria, 2017-07-21 03:07 (2865 d 17:45 ago) – Posting: # 17587
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Hi John,

❝ Is calcitriol bioanalytical method a really difficult method?


Define difficult. ;-)
An LLOQ of 15 pg/mL will do the job. That’s doable with a validated (!) commercial RIA.

IMHO, the design recommended in the FDA’s guidance is problematic. I don’t get the point of the FDA’s 0, –6, –12, and –18 hours sampling. Averaging (as suggested) IMHO, is crap. There is a circadian rhythm of basal levels (24 ♂ x 30–50 pg/mL). See also Rejnmark et al. 2002.*
I recommend full basal profiles in each period. A dose of 0.5 µg will be extremely difficult. Not analytically but you will be very close to basal levels. Our Cmax (unadjusted geometric mean) was 222 pg/mL after a 4 µg dose. CVintra of Cmax,adj was 27% and of AUC0–t,adj 20%. Given that, after a 0.5 µg dose you could expect an unadjusted Cmax of ~62 pg/mL at a time when basal levels are ~39 pg/mL. That’s difficult. I guess you will run into serious troubles estimating λz.
I know of another study with a 2 µg dose, where (with averaging as suggested by the FDA) the CRO only by some acrobatic means could deal with the later part of the profile. The unadjusted Cmax was 134 µg/mL and Cmax,adj 76 µg/mL (25 ♀♂).
Im pretty sure that at least the CV of Cmax will be >30% and and therefore, a candidate for RSABE. I don’t expect that demonstrating BE with a 0.5 µg dose is realistic at all. Talk to the OGD.



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