Modified polysaccharide compositions and their use in combination with at least one anti-cancer drug for treating subjects with cancer, reduce toxicity and inhibit metastasis, are described. The modified polysaccharide includes a saccharide backbone being less than 5% esterified and containing repeating units, wherein each repeating unit has a plurality of uronic acid molecules, each repeating unit having at least one neutral monosaccharide attached thereto, at least one side chain of saccharides attached to the backbone further comprising a plurality of neutral saccharides or saccharide derivatives; and having an average molecule weight in the range of 15 to 60 kD. The polysaccharide when combine with the chemotherapeutic drug behave as a delivery vehicle, which positively enhance the chemotherapeutic effect while reducing side effects.