Bioequivalence and Bioavailability Forum

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 C_max (unadjusted geometric mean) was 222 pg/mL after a 4 µg dose. CV_intra of C_max,adj was 27% and of AUC_0–t,adj 20%. Given that, after a 0.5 µg dose you could expect an unadjusted C_max 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 λ_z.
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 C_max was 134 µg/mL and C_max,adj 76 µg/mL (25 ♀♂).
Im pretty sure that at least the CV of C_max 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.

---

• Rejnmark L, Lauridsen AL, Vestergaard P, Heickendorff L, Andreasen F, Mosekilde L. Diurnal rhythm of plas­ma 1,25-dihydroxyvitamin D and vitamin D-binding protein in postmenopausal women: relationship to plasma parathyroid hormone and calcium and phosphate metabolism. Eur J Endocrin. 2002;146:635-42. doi:10.1530/eje.0.1460635