Gjestad et al. reply to ‘Was 4β‐hydroxycholesterol ever going to be a useful marker of CYP3A4 activity?’ by Neuhoff and Tucker

We thank Dr Neuhoff and Prof Tucker for paying attention to our recent paper presenting data on a highly significant correlation between 4β‐hydroxycholesterol (4βOHC) level and dose‐corrected serum concentration of http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=50 1. Neuhoff and Tucker express their fundamental skepticism to 4βOHC as a http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1337 biomarker 2, which they consider to be supported by the two papers in the same issue of the journal reporting correlation for one CYP3A substrate (quetiapine; our study 1), but not the other (http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=6784; study of Vanhove et al. 3). However, a key point is that various drugs subjected to metabolism/elimination via the same enzyme/pathway, in this case CYP3A, may completely differ in terms of other pharmacokinetic characteristics. While CYP3A4 metabolism and P‐glycoprotein (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=768) transport are key mechanisms determining quetiapine exposure 1, distribution to red blood cells – and hence haematocrit – is an additional factor of major importance for tacrolimus exposure 4.

In addition to the pharmacokinetic differences between quetiapine and tacrolimus, the type of population and design was different between our study 1 and the study by Vanhove et al. 3. In the latter, pretransplant 4βOHC levels were matched with post‐transplant tacrolimus concentrations. We recently showed that 4βOHC levels increase following kidney transplantation due to regained CYP3A4 activity 5, which is suppressed in end‐stage renal disease 6. It is therefore no surprise that pretransplant 4βOHC levels did not correlate with post‐transplant tacrolimus concentrations in the study by Vanhove et al. 3. However, in a previous study published in this journal, Vanhove et al. showed that post‐transplant 4βOHC levels correlate with dose‐corrected tacrolimus concentration when renal function has stabilized 7. In the same study, it was also shown that cholesterol‐ and weight‐adjusted 4βOHC level is superior to http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=3342 clearance in explaining variability in tacrolimus concentration 7. The limited correlation between 4βOHC level and midazolam clearance, reported in a study by Tomalik‐Scharte et al. 8, is hence of secondary importance, as these CYP3A4 metrics likely perform differently for various substrates dependent on other pharmacokinetic characteristics than CYP3A4‐mediated metabolism.

4βOHC is an endogenous CYP3A4 metabolite, which has been shown to be sensitive towards both administration of inducers and inhibitors of CYP3A4 9, 10, 11, 12, 13, 14, and not only the enzyme inducer http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2765, which Neuhoff and Tucker refer to 2. To show a potential reduction in 4βOHC level, treatment duration of the inhibitor should be several weeks due to the long elimination half‐life of 4βOHC 13. In experimental studies on healthy subjects, use of the potent CYP3A4 inhibitors atazanavir/http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=8804 and itraconazole or http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2568 significantly reduced 4βOHC levels by approximately 40% 12 and 20–30% 10, 14, respectively. More recently, it was shown that patients on long‐term treatment with the moderate CYP3A4 inhibitors http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=7189 or http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=203 display 30–35% lower concentration of 4βOHC compared to those without CYP3A4‐interacting drugs 11. Although 4βOHC is more sensitive in detecting CYP3A4 induction than inhibition 13, it is therefore clear that 4βOHC also responds to CYP3A4 inhibition when treatment duration of the inhibitor is long enough for 4βOHC to reach a new steady‐state level.

In addition to its potential of detecting CYP3A4 induction or inhibition, the endogenous nature of 4βOHC opens the possibility for it to be included as a non‐invasive biomarker in dosing algorithms of drugs metabolized by CYP3A4. In our study, we found that the serum concentration of 4βOHC together with age and sex explained ~30% of the interindividual variability in dose‐corrected steady‐state trough (single‐point) quetiapine concentration. For quetiapine, a drug with low oral bioavailability (~10%) and short elimination half‐life (~5–7 h), the explanation degree is actually fairly good. Although this result does not provide a basis for individualized dosing of quetiapine solely using information about 4βOHC concentration, age and sex, it may represent a start for establishing 4βOHC‐based algorithms that can be tested for the ability to predict dose requirements of quetiapine and possibly other CYP3A4 substrates.

No clinical studies have so far implemented measurements of midazolam clearance or 4βOHC level for pre‐emptive estimations of dose requirements of drugs metabolized by CYP3A4. Future, prospective studies are necessary to evaluate the clinical potential of these, and possibly other biomarkers, for individualized dosing of CYP3A4 substrates.

Nomenclature of targets and ligands

Key protein targets and ligands in this article are hyperlinked to corresponding entries in http://www.guidetopharmacology.org, the common portal for data from the IUPHAR/BPS Guide to PHARMACOLOGY 15, and are permanently archived in the Concise Guide to PHARMACOLOGY 2017/18 16, 17.

Competing Interests

There are no competing interests to declare.

We acknowledge the South‐Eastern Norway Regional Health Authority for PhD funding to author C.G. (#2014115).

Gjestad, C. , Haslemo, T. , Andreassen, O. A. , and Molden, E. (2018) Gjestad et al. reply to ‘Was 4β‐hydroxycholesterol ever going to be a useful marker of CYP3A4 activity?’ by Neuhoff and Tucker. Br J Clin Pharmacol, 84: 1624–1625. doi: 10.1111/bcp.13606.