Oncogenic osteomalacia (OOM) is a
rare disease characterized by renal
phosphate wasting and
osteomalacia and is caused by the secretion of
fibroblast growth factor 23 (FGF-23) from causative
tumors. Scintigraphy with
octreotide, which binds to
somatostatin receptors (SSTRs), is a useful way to locate causative
tumors in OOM patients. However, the
therapeutic effects of
octreotide acetate are still controversial. Two OOM patients were administered
octreotide acetate intramuscularly. Ten causative OOM
tumors, including two resected from the patients participating in the
octreotide administration study, were examined for expression of genes encoding SSTRs by quantitative real-time RT-PCR and immunohistochemistry.
Octreotide therapy did not improve
hypophosphatemia in either case, despite temporal decreases in FGF-23 levels in one patient. The mean expression levels of
SSTR1, SSTR3, and SSTR5 were similar in the OOM and non-OOM
tumors. Expression of SSTR2 was significantly higher in the OOM
tumors than in the non-OOM
tumors. Immunohistochemical examinations revealed the presence of
SSTR2A, SSTR2B, and SSTR5 in both the OOM and non-OOM
tumors. The expression of SSTR genes in OOM
tumors contributes to positive imaging using
octreotide scintigraphy. However, the levels of SSTRs seem to be insufficient for the
octreotide therapy to improve
hypophosphatemia. Further studies are needed to clarify the mechanisms by which FGF-23 secretion from OOM
tumors is suppressed by
octreotide acetate.