Pathogenesis
FGF23 (fibroblast growth factor 23) inhibits phosphate transport in the renal tubule and reduces calcitriol production by the kidney. Tumor production of FGF23, Secreted frizzled-related protein 4 and matrix extracellular phosphoglycoprotein (MEPE) have all been identified as possible causative agents for the hypophosphatemia. Diagnosis
Biochemical studies reveal hypophosphatemia (low blood phosphate), elevated alkaline phosphatase and low serum 1,25 dihydroxyvitamin D levels. Routine laboratory tests may not include serum phosphate levels and this can result in considerable delay in diagnosis. Even when low phosphate is measured, its significance is often overlooked. The next most appropriate test is measurement of urine phosphate levels. If there is inappropriately high urine phosphate (phosphaturia) in the setting of low serum phosphate (hypophosphatemia), there should be a high suspicion for tumor-induced osteomalacia. FGF23 (see below) can be measured to confirm the diagnosis but this test is not widely available. Once hypophosphatemia and phosphaturia have been identified, begin a search for the causative tumor, which may be small and difficult to detect. Gallium-68 DOTA-Octreotate (DOTA-TATE) positron emission tomography (PET) scanning is the best way to locate these tumors. If this scan is not available, other options include Indium-111 Octreotide (Octreoscan) SPECT/CT, whole body CT or MRI imaging. Differential diagnosis
Serum chemistries are identical in tumor-induced osteomalacia, X-linked hypophosphatemic rickets (XHR) and autosomal dominant hypophosphatemic rickets (ADHR). A negative family history can be useful in distinguishing tumor induced osteomalacia from XHR and ADHR.
Asciminib, sold under the brand name Scemblix, is a medication used to treat Philadelphia chromosome-positive chronic myeloid leukemia (Ph+ CML). Asciminib is a protein kinase inhibitor.The most common adverse reactions include upper respiratory tract infections, musculoskeletal pain, fatigue, nausea, rash, and diarrhea.Asciminib was approved for medical use in the United States in October 2021, and in the European Union in August 2022.The U.S. Food and Drug Administration (FDA) granted the application for asciminib priority review, fast track, orphan drug, and breakthrough therapy designations. Mechanism of action
Asciminib is described as a "STAMP inhibitor," which means "specifically targeting the ABL myristoyl pocket." The wild-type ABL has a myristoylated N-terminus, which binds to an allosteric site, but the ABL fusion protein does not have the myristoylated domain. In the wild-type protein, when myristoylated N-terminus binds to the allosteric site, the kinase has reduced activity. Since the mutant fusion protein does not have the myristoylated N-terminus domain, it is not subject to this form of regulation, and thus the fusion protein is constitutively active. Asciminib binds to the allosteric site, resulting in an inhibition of bcr-abl activity.Unlike other bcr-abl inhibitors, such as imatinib, asciminib does not bind to the ATP-binding site on the active site of the enzyme. Asciminib and active site bcr-abl inhibitors have non-overlapping resistance mutations. The mutations A337V and P223S overcome the inhibitory activity of asciminib, but asciminib is not affected by the notorious T315I mutation that affects most ATP-competitive active site inhibitors, except ponatinib.