Lanthanum compounds are provided having general formulas (I) R.sub.n.sup.i+Y.sub.i.sup.n- and (II) R.sub.b.sup.+Y.sub.b.sup.- which may be used as medicaments in the prophylaxis and/or treatment of cancer diseases. In formula (I), R is a group of general formula (A): ##STR00001## and in general formula (II) R.sub.b is a group of general formula (B): ##STR00002##