Discussion
NET
Ours is the first study to determine baseline and peak serum concentrations of MPA or NET from a relatively large cohort of women randomized to DMPA-IM (n = 215) or NET-EN (n = 220). This is the most robust data to date on these levels for each contraceptive as well as their comparative values in a head-to-head comparison. We report for the first time the concentrations of study and non-study progestins measured for this relatively large cohort of women at baseline (D0) and at one week after a six-month injection (25W) to allow robust assessment of non-study progestins in a randomized trial at both sampling times, as well as subgroup analysis after exclusion of some participants with non-study or no study progestins at 25W. This comprehensive dataset allowed us to validate our prediction of peak study progestin serum levels occurring at one week after injection, as well as to detect significant associations within and between contraceptive arms, between progestin concentrations and BMI and weight for participants.We report a median peak serum concentration of MPA for DMPA-IM users at 25W of 6.59 nM (2.55 ng/mL) (IQR 4.80–8.70 nM) for the whole cohort of 215 women (mITT group). A median peak serum MPA concentration of 6.83 nM (2.64 ng/mL) (IQR 5.09–8.80 nM) was obtained for the subgroup of 161 women (PP1 group) after excluding some participants with any quantifiable non-study progestins at 25W. Many different studies have previously investigated peak serum concentrations for MPA in DMPA-IM users, some having measured at multiple time points, and using different methods including radioimmunoassay and LC-MS/MS (reviewed in [When comparing our results for both contraceptive arms, we report that the peak serum concentration for MPA of 6.83 nM is about 2-fold lower than the 13.35 nM value for NET, which may be partly due to the 1.26-fold lower dose in mmoles for DMPA-IM (150 mg, 0.388 mmoles) than NET-EN (200 mg, 0.487 mmoles). Levene’s test for differences in variance indicated that the inter-individual variability of MPA concentration (median 6.83 nM and range 0.168 nM to 20.4 nM) in the DMPA-IM arm is less than that for NET concentrations (median 13.3 nM and range 2.42 nM to 52.9 nM) in the NET-EN arm in our subgroup. Our findings are also unlikely to be confounded by differences in baseline characteristics between contraceptive arms as we do not detect any significant differences between demographic factors at baseline for the subgroups (PP1 or PP2 analysis), and only detect significant differences at baseline for two factors in the whole cohort (It is important to consider the implications of our peak serum progestin concentration results regarding steroid receptor effects. Both MPA and NET concentrations are clearly sufficient to result in PR-mediated responses since both DMPA-IM and NET-EN use result in efficient contraceptive effects which are due to their progestogenic actions via the PR. What is more difficult to assess is whether peak serum MPA at around 6.59 nM will result in GR-mediated side-effects MPA was the most commonly quantifiable non-study progestin at initiation in both arms (quantifiable in 54.3% of women) and at 25W in the NET-EN arm (quantifiable in 27.3% of women), followed by NET at D0 in both arms (28.5% of women) and at 25W in the DMPA-IM arm (18.6% of women), and then LNG for both arms at D0 (6.90% of women) and at 25W (3.40% of women) (Our results regarding the presence of non-study progestins are in accordance with a previous study which found that among 358 women reporting different forms of hormonal contraceptive use, including DMPA-IM and NET-EN, 28% of samples were not fully concordant with self-report, as were 14% of samples from 744 women reporting no hormonal contraceptive use [We show for the first time in a highly powered randomized study that the greater the BMI and weight at D0, the lower the progestin serum concentration at 25W, as well as the smaller the change from D0. Our data are consistent with findings from progestin-only LNG and ETG implant studies [Associations between contraceptive use and health outcomes or changes in biological markers from observational clinical trials are likely to be confounded by multiple factors, including differences at baseline in demographic factors, especially BMI and weight, numbers of participants misreporting contraceptive use, a lack of objective validation of progestin levels at initiation and during the study, variations in sampling time and the use of inter-study comparisons. Randomized trials comparing contraceptives likely avoid some but not all of these potentially confounding factors. To the best of our knowledge, all published health outcomes and biological marker data thus far from clinical trials on contraceptives have not taken into account the possible confounder of non-study progestins at initiation and during follow-up when drawing conclusions, with the notable exception of data from the non-randomized Zim CHIC study [