Activity of dendrimer-methotrexate conjugates on methotrexate-sensitive and -resistant cell lines

Dendritic nanostructures can play a key role in drug delivery, due to the high density and variety of surface functional groups that can facilitate and modulate the delivery process. We have investigated the effect of dendrimer end-functionality on the activity of polyamido amine (PAMAM) dendrimer-methotrexate (MTX) conjugates in MTX-sensitive and MTX-resistant human acute lymphoblastoid leukemia (CCRF-CEM) and Chinese hamster ovary (CHO) cell lines. Two amide-bonded PAMAM dendrimer-MTX conjugates were prepared using a dicyclohexylcarbodiimide (DCC) coupling reaction: one between a carboxylic acid-terminated G2.5 dendrimer and the amine groups of the MTX (conjugate A) and another between an amine-terminated G3 dendrimer and the carboxylic acid group of the MTX (conjugate B). Our studies suggest that conjugate A showed an increased drug activity compared to an equimolar amount of free MTX toward both sensitive and resistant cell lines, whereas conjugate B did not show significant activity on any of the cell lines. Despite substantially impaired MTX transport by MTX-resistant CEM/MTX and RII cells, conjugate A showed sensitivity increases of approximately 8- and 24-fold (based on IC50 values), respectively, compared to free MTX. Co-incubation of the cells with adenosine and thymidine along with either conjugate A or MTX resulted in almost complete protection, suggesting that the conjugate achieves its effect on dihyrofolate reductase (DHFR) enzyme through the same mechanism as that of MTX. The differences in cytotoxicity of these amide-bonded conjugates may be indicative of differences in the intracellular drug release from the cationic dendrimer (conjugate B) versus the anionic dendrimer (conjugate A), perhaps due to the differences in lysosomal residence times dictated by the surface functionality. These findings demonstrate the feasibility of using dendrimers as drug delivery vehicles for achieving higher therapeutic effects in chemotherapy, especially in drug-resistant cells.