Yes, this is a real thing
In brief: 503A pharmacies are state-licensed compounding pharmacies that prepare medications based on individual patient prescriptions
Biosci Rep 17:293302, PMID: 9337484 Sato T, Machida T, Takahashi S, Iyama S, Sato Y, Kuribayashi K, Takada K, Oku T, Kawano Y, Okamoto T, Takimoto R, Matsunaga T, Takayama T, Takahashi M, Kato J, Niitsu Y (2004) Fas-mediated apoptosome formation is dependent on reactive oxygen species derived from mitochondrial permeability transition in Jurkat cells
Elsevier, Oxford, pp 175215 Rausch S, Gouttefangeas C, Hennenlotter J, Laske K, Walter K, Feyerabend S, Chandran PA, Kruck S, Singh-Jasuja H et al (2019) Results of a phase 1/2 study in metastatic renal cell carcinoma patients treated with a patient-specific adjuvant multi-peptide vaccine after resection of metastases