An aqueous composition comprising particles for use in the treatment, reduction, amelioration, or prevention of cancer growth, metastasis, ulceration, epithelial-mesenchymal transition, immunologic escape or recurrence in a subject, wherein: (a) each particle comprises a complex of at least one double-stranded polyribonucleotide, or a salt or solvate thereof, and at least one polyalkyleneimine, or a salt and/or solvate thereof, wherein (i) the double-stranded polyribonucleotides are polyinosinic-polycytidylic acid [poly(I:C)] molecules, wherein at least 60% of the double-stranded polyribonucleotides have at least 850 base pairs, at least 70% of the double-stranded polyribonucleotides have between 400 and 5000 base pairs, and between 20% and 45% of the double-stranded polyribonucleotides have between 400 and 850 base pairs; and (ii) the polyalkyleneimine comprises at least 95% linear polyethyleneimines; wherein the weight average molecular weight of the polyalkyleneimine is between 17 and 23 kDa and the molecular weight polydispersity index is < 1.5, and wherein the ratio of the number of moles of nitrogen of the polyalkyleneimine to the number of moles of phosphorus of the poly(I:C) molecules in the composition is between 2.5 and 5.5; (b) the particles have a z-average diameter measured according to ISO 22412:2008 of between 30 nm and 150 nm, (c) at least 99% of the particles have a diameter distribution of less than or equal to 600 nm, (d) said aqueous composition has a pH of between 2 and 4 and an osmolality of between 200 and 600 mOsm/kg, (e) said composition has a zeta potential of between 35 and 50 mV, according to ISO 13099-2:2012, and wherein said composition is administered in combination with one or more immune-modulating agents, wherein said agent is a monoclonal antibody that targets PD-1, PD-L1, PD-L2, CTLA-4, CD137, OX-40, TIM3, or LAG3. (a) each particle comprises a complex of at least one double-stranded polyribonucleotide, or a salt or solvate thereof, and at least one polyalkyleneimine, or a salt and/or solvate thereof, wherein (i) the double-stranded polyribonucleotides are polyinosinic-polycytidylic acid [poly(I:C)] molecules, wherein at least 60% of the double-stranded polyribonucleotides have at least 850 base pairs, at least 70% of the double-stranded polyribonucleotides have between 400 and 5000 base pairs, and between 20% and 45% of the double-stranded polyribonucleotides have between 400 and 850 base pairs; and (ii) the polyalkyleneimine comprises at least 95% linear polyethyleneimines; wherein the weight average molecular weight of the polyalkyleneimine is between 17 and 23 kDa and the molecular weight polydispersity index is < 1.5, and wherein the ratio of the number of moles of nitrogen of the polyalkyleneimine to the number of moles of phosphorus of the poly(I:C) molecules in the composition is between 2.5 and 5.5; (i) the double-stranded polyribonucleotides are polyinosinic-polycytidylic acid [poly(I:C)] molecules, wherein at least 60% of the double-stranded polyribonucleotides have at least 850 base pairs, at least 70% of the double-stranded polyribonucleotides have between 400 and 5000 base pairs, and between 20% and 45% of the double-stranded polyribonucleotides have between 400 and 850 base pairs; and at least 60% of the double-stranded polyribonucleotides have at least 850 base pairs, at least 70% of the double-stranded polyribonucleotides have between 400 and 5000 base pairs, and (ii) the polyalkyleneimine comprises at least 95% linear polyethyleneimines; wherein the weight average molecular weight of the polyalkyleneimine is between 17 and 23 kDa and the molecular weight polydispersity index is < 1.5, and wherein the ratio of the number of moles of nitrogen of the polyalkyleneimine to the number of moles of phosphorus of the poly(I:C) molecules in the composition is between 2.5 and 5.5; (b) the particles have a z-average diameter measured according to ISO 22412:2008 of between 30 nm and 150 nm, (c) at least 99% of the particles have a diameter distribution of less than or equal to 600 nm, (d) said aqueous composition has a pH of between 2 and 4 and an osmolality of between 200 and 600 mOsm/kg, (e) said composition has a zeta potential of between 35 and 50 mV, according to ISO 13099-2:2012, and