The cytotoxic potential of three cucurbitacins, 2,3,16,20(R),25-pentahydroxy-11,22-dioxo-cucurbita-5-en (cucurbitacin P, 1), 2,3,16,20(R),25-pentahydroxy-22-oxocucurbita-5-en (2) and 2,3,16,20(R),25-pentahydroxy-22-oxocucurbita-5,23(E)-diene (deacetylpicracin, 3), obtained from Cayaponia racemosa