M: Musculo-skeletal system
Drugs for joint, bone and muscle disease, plus the neuromuscular blockers given under anesthesia [1]. Published models here span a subjective pain score [2] and an evoked muscle twitch [1].
In 1938 Richard Gill returned from the Amazon with 11 kg of curare, from which Squibb isolated a standardized extract; Harold Griffith and Enid Johnson gave it in the operating room in 1942 [3,4].
| Level 2 code | Description | In plain terms | Example drug |
|---|---|---|---|
| M01 | Antiinflammatory and antirheumatic products | Ibuprofen (M01AE01) / naproxen (M01AE02) | |
| M02 | Topical products for joint and muscular pain | Topical diclofenac (M02AA15) / topical ketoprofen (M02AA10) | |
| M03 | Muscle relaxants | Rocuronium bromide (M03AC09) for neuromuscular block / baclofen (M03BX01) for spasticity | |
| M04 | Antigout preparations | Allopurinol (M04AA01) / febuxostat (M04AA03) | |
| M05 | Drugs for treatment of bone diseases | Alendronic acid (M05BA04) / denosumab (M05BX04) | |
| M09 | Other drugs for disorders of the musculo-skeletal system | leftover bone, joint and muscle drugs | Residual catch-all group, but it now carries real drug-level standards for rare neuromuscular disease: nusinersen (M09AX07) and risdiplam (M09AX10) for spinal muscular atrophy |
Modeling notes
Endpoint. Each level 2 code brings its own: WOMAC pain in M01 [5], serum urate in M04 [6], bone markers and density in M05 [7], evoked twitch in M03 [8].
Easy to overlook. For M02 the relevant exposure is local, in skin and synovial fluid, and dermal PBPK is what links measured plasma concentration to that site, so bioequivalence is argued at the site of action alongside, not instead of, a plasma study [9].
Mechanistic extrapolation. A multiscale model of calcium homeostasis and bone remodeling [10] was extended to predict lumbar spine density under denosumab, including the decline after discontinuation [11]. For M02, dermal PBPK replaced a comparative clinical endpoint study in an FDA generic approval of topical diclofenac gel [9].