AlprostadilvsHMG

Alprostadil is synthetic prostaglandin E1 (PGE1), a 20-carbon oxygenated fatty acid derived from dihomo-gamma-linolenic acid (DGLA) through the cyclooxygenase pathway. It acts locally on penile vascular and trabecular smooth muscle through two prostaglandin E receptor subtypes: EP2 and EP4, both of which are Gs-coupled GPCRs that increase intracellular cAMP upon activation.

Elevated cAMP activates protein kinase A (PKA), which phosphorylates multiple targets in smooth muscle cells to produce relaxation. PKA phosphorylates myosin light chain kinase (MLCK), reducing its affinity for the calcium-calmodulin complex and decreasing its ability to phosphorylate myosin light chains — the final step in smooth muscle contraction. PKA also activates calcium-ATPase pumps and opens potassium channels, reducing intracellular calcium concentration. The net effect is relaxation of both the helicine arterioles (which supply blood to the corpora cavernosa) and the trabecular smooth muscle (which forms the spongy erectile tissue). As these relax, blood flows into the sinusoidal spaces of the corpora cavernosa, expanding the tissue against the tunica albuginea and compressing the subtunical veins — trapping blood and producing an erection.

The critical distinction of alprostadil's mechanism is its direct, local action independent of central sexual arousal pathways and independent of nitric oxide. PDE5 inhibitors (sildenafil, etc.) work by preventing cGMP breakdown downstream of nitric oxide release, which requires sexual arousal to generate the initial NO signal. Alprostadil generates its own second messenger (cAMP) at the injection site regardless of arousal state, which is why it produces erections reliably even in patients with neurogenic erectile dysfunction (spinal cord injury, radical prostatectomy) where the nerve-mediated NO pathway is damaged. The extremely rapid pulmonary metabolism (80% cleared in a single pass through the lungs) ensures that systemic effects are minimal when administered locally.

Human Menopausal Gonadotropin is a purified urinary extract containing both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) activity, sourced from the urine of postmenopausal women. After menopause, the loss of ovarian negative feedback (estradiol and inhibin) results in dramatically elevated pituitary gonadotropin secretion — FSH and LH levels rise 10-20 fold, providing a natural source of these hormones for pharmaceutical extraction.

The FSH component binds to FSH receptors (FSHR) on Sertoli cells in males and granulosa cells in females. FSHR is a Gs-coupled GPCR that activates cAMP/PKA signaling, driving the expression of genes essential for gametogenesis. In males, FSH-stimulated Sertoli cells produce androgen-binding protein (which concentrates testosterone locally), inhibin B (which provides negative feedback to the pituitary), and multiple growth factors that support spermatogonial proliferation and differentiation through the stages of spermatogenesis. In females, FSH drives follicular development — stimulating granulosa cell proliferation, estradiol synthesis via aromatase induction, and the growth of ovarian follicles from the pre-antral to the pre-ovulatory stage.

The LH component acts on Leydig cells in males (stimulating testosterone production via the LHCGR/cAMP/StAR steroidogenic pathway) and on theca cells in females (stimulating androgen precursor production that granulosa cells convert to estradiol). In females undergoing fertility treatment, the LH component is also critical for final oocyte maturation and ovulation triggering. The combination of both FSH and LH activity in HMG provides more complete gonadal stimulation than either gonadotropin alone — FSH drives the cellular proliferation and maturation processes while LH provides the steroidogenic and final maturation signals. This dual activity is why HMG is sometimes preferred over purified FSH preparations in certain fertility protocols, particularly in hypogonadotropic patients who lack endogenous LH.