HXR9 is a cell-permeable peptide that acts as a competitive antagonist of the HOX/PBX interaction, effectively inhibiting the binding between HOX proteins and the transcription factor PBX in paralogue groups 1 to 8. HXR9 selectively impairs cell proliferation and induces apoptosis in cells with high expression of the HOXA/PBX3 genes (e.g., MLL-rearranged leukemic cells).

HXR9 at a concentration of 60μM effectively disrupts the interaction between PBX and HOX within 4 hours, and at the same duration, it also blocks the binding of HOXD9 to PBX in murine B16 melanoma cells as confirmed by Western Blot Analysis. Additionally, a 2-hour treatment with HXR9 induces apoptosis in B16 and primary melanoma cells, with a significant increase in late apoptosis phases observed in B16 cells. This compound also prompts specific transcriptional alterations, notably the upregulation of Fos, Jun, Dusp1, and Atf1 as demonstrated by RT-PCR in B16F10 cells. Moreover, HXR9 exhibits antiproliferative effects on B16 cells, with an IC50 value of 20μM, highlighting its potential as a therapeutic agent in melanoma treatment.

HXR9, administered intravenously (i.v.) via the tail vein at a dosage of 10 mg/kg twice weekly, effectively inhibits tumor growth, as evidenced in C57black/6 mice bearing B16 cells, leading to significant retardation of tumor progression over approximately 30 days. Similarly, when commenced with an initial dose of 100 mg/kg followed by a maintenance dosing of 10 mg/kg twice weekly via intraperitoneal injection for 18 days, HXR9 substantially suppresses A549 tumor growth in vivo in athymic nude mice, resulting in markedly smaller tumors compared to those in control groups. This consistent anti-tumor activity of HXR9 underscores its potential in treating different tumor types across various animal models.