HPK1-IN-22 (compound ZYF0033) is a haematopoietic progenitor kinase 1 (HPK1) inhibitor with an IC50 of less than 10 nM for MBP protein-based phosphorylation inhibition. HPK1-IN-22 reduces phosphorylation of SLP76 (serine 376) and promotes anti-cancer immune responses. HPK1-IN-22 inhibits tumor growth and increases intra-tumor infiltration of DCs, NK cells and CD107a+CD8+ T cells, but decreases infiltration of T cells, PD-1+CD8+ T cells, TIM-3+CD8+ T cells and LAG3+CD8+ T cells in a 4T-1 homozygous mouse model.