Primo is virtually interchangeable functionally with Mast. Both are relatively modest in their potency to transactivate mammalian AR, but Mast > Primo actually in this potency (33.2% the potency of DHT [Mast] vs. 28.75% the potency of DHT [Primo] per Houtman). Both are unfortunately subject to metabolism (breakdown in human skeletal muscle) by 3α-HSD, resulting in dramatically reduced skeletal muscle anabolism in human vs. rodent. Indeed, this similarity is shared by Proviron, and as such it is not unreasonable to mention Proviron (a far more potent AR agonist on paper than Mast or Primo) in the same discussion.
Where the two seem to differ functionally, is that Mast has been shown to prevent the uptake of estrogens into, e.g. breast cells, and is therefore a tissue-specific modulator of estrogenic activity (not unlike epitiostanol, the non-methylated counterpart of Epistane that is used clinically in Japan to treat gynecomastia). Primo may serve to reduce circulating estrogens by 17β-HSD1 inhibition (DHT has been shown to be a good ligand for this enzyme), and metenolone's more saturated/estrogen-like A-ring may confer some inhibitory potency for 17β-HSD1 [resulting in decreased E2]), and/or aromatase inhibition. There is insufficient evidence to state either with confidence presently. What matters is that both have good efficacy and attenuated androgenicity particularly in treatment-resistant advanced cancer of the breast. Mast fell out of favor because of its significantly greater androgenicity to Primo in women, however; hence the actual reason why it's not available commercially as a pharmaceutical preparation.
It's great that you are a student of medicine, I wish you all the best brother. Do be aware that bold confidence on this forum, as a practical AAS neophyte, will require that you do more than make appeals to authority of ongoing education, however. And do avoid the perils (I am not saying you have done so, but I think I see a tendency) of conflating absence of evidence with evidence of absence, a common logical pitfall or fallacy.
I disagree that Primo iS iNcOmPrEnSiBlY nOnCoMpArAbLe to Mast in practice. They are remarkably similar. While I think most do prefer Primo, as I do; this is largely because it's appealing to me that it has known advantageous cardiovascular effects versus testosterone (but absence of evidence for Mast is not evidence of absence for these effects), and that it is still commercially available in some markets as a pharmaceutical product. It does not follow that Primo is superior per se to Mast, however.
Indeed, if you investigate Primo (metenolone enanthate)'s safety profile using a tool like FAFDrugs4tool, you may be surprised to learn that it fails to comply with the GlaxoSmithKline 4/400 Rule, Lilly Med Chem Rules, in addition to Pfizer's 3/75 Rule on logP, and is thus regarded as an unsafe and therefore unapproved medicine under many international and national standards.
