The present invention provides methods for treating cancer and other pathological proliferating conditions by inhibiting mitosis using at least one pyrrolo[2,3-d]pyrimidine having the general formula (16):where X is selected from the group consisting of lower alkyls, heteroalkyls, substituted or unsubstituted aryls or heteroaryls, arylalkyls, and heteroarylalkyls where R1 is selected from the group consisting of hydrogen, lower alkyls, heteroalkyls, substituted or unsubstituted aryls or heteroaryls, arylalkyls, and heteroarylalkyls where R2 is selected from the group consisting of hydrogen, lower alkyls, heteroalkyls, alkoxys, substituted or unsubstituted aryls or heteroaryls where R3 is selected from the group consisting of zero, lower alkyls, heteroalkyls, alkenyls, and heteroalkenyls and where R4 is selected from the group consisting of substituted or unsubstituted aryls or heteroaryls, arylalkyls, heteroarylalkyls, and hydrogen. The compound may inhibit mitosis in cells that have developed multidrug resistance due to P-glycoprotein and MRP1, and facilitate the reversal of P-glycoprotein mediated resistance.
