Inhibiting the IGF-1 Receptor Tyrosine Kinase with Picropodophillin: An In Vitro and In Vivo Study in the 5T33MM Mouse Model.

Multiple myeloma (MM) represents a B-cell malignancy, characterized by monoclonal proliferation of plasma cells in the bone marrow (BM) and is associated with osteolysis and angiogenesis. Insulin-like growth factor-1 (IGF-1), produced by the BM stromal cells, has been described as an important factor in the survival, proliferation and migration of MM cells. The latter process is involved in the homing of the MM cells to the BM. IGF-1 also induces VEGF secretion by the MM cells, thus stimulating angiogenesis in the BM. As IGF-1 is a pleiotropic factor in MM, therapeutic strategies targeting the IGF-1R may be effective as anti-tumor treatments. In this work we investigated the effect of an IGF-1 receptor tyrosine kinase inhibitor (picropodophyllin or PPP1) in the murine, syngeneic 5T33MM model of multiple myeloma. This mouse model is representative for the human disease and can combine in vitro and in vivo studies.

We first investigated the effects of PPP on the MM cells in vitro. We and others have previously demonstrated that IGF-1 induced ERK activation, involved in VEGF secretion and proliferation. When the 5T33MM cells were preincubated with 1microM PPP, Western blot analysis demonstrated the blocking of this activation. Furthermore, when the 5T33MM cells were preincubated with PPP for 30 min, IGF-1 induced VEGF secretion and proliferation of the 5T33MM cells were completely blocked.

Next, we used the tyrosine kinase inhibitor PPP in vivo. 5T33MM cells were injected intravenously in C57BLKaLwRij mice and the development of the disease was monitored by measuring the serum paraprotein concentration. Mice were either treated with a low (17mM, IP, twice a day) or a high dose of PPP (50mM, IP, twice a day) or with the vehicle (DMSO/oil 9/1) from the day of injection with 5T33MM onward. At week 3, vehicle controls showed signs of morbidity and were sacrificed. The presence of tumor was measured by assessing serum paraprotein concentrations and determining the proportion of idiotype positive cells in the BM by flow cytometry. Angiogenesis was assessed by measuring the microvessel density on CD31 stained paraffin sections. The tumor burden in the bone marrow in the PPP treated mice was 77% lower than in vehicle treated animals (p< 0,0001) and the serum paraprotein concentration was 90% lower (p< 0,0001). The microvessel density in the BM of the PPP treated group was reduced by 60% (p< 0,02). In a separate survival experiment the mice were either treated with the vehicle or with the high dose (50mM) of PPP, from the time of tumor injection. Kaplan-Meier analysis demonstrated a significant increase in survival after treatment with PPP when compared with vehicle (28 vs. 18 days, p<0,001).

These data demonstrate that the IGF-1RTK inhibitor PPP possesses strong anti-tumor activity, as demonstrated both in vitro and in vivo in a syngeneic model of multiple myeloma, and may therefore be an effective therapeutic candidate for MM treatment.