Abstract
Background:
Five plant extracts (Orthosiphon stamineus, Morus nigra, Averrhoa bilimbi, Punica granatum, and Eurycoma longifolia) were studied in this work for antioxidant activities using ABTS and DPPH; the two most potent extracts in terms of antioxidant activity were further studied for antiproliferative activity against cancer cell lines.

Aim:
To study the antiproliferative activity of two potent extracts at different combination ratios on two cancer cell lines (HepG2 and A375) using the MTT assay.

Methods:
About 20 g of the dried and pulverized plant materials were extracted with 200 mL of 80 % ethanol in an ultrasonic homogenizer for 15 minutes (using 15-second pulse-on and 5-second pulse-off period). The resulting extract was filtered and concentrated in a rotary evaporator to 20 mL before freeze drying to eliminate alcohol content. The phytochemical tests were done using several chemical methods. Antioxidant activity of the extracts was assessed using ABTS and DPPH methods. Two plant extracts (O. stamineus and P. granatum extracts) with the highest antioxidant activities out of the screened five extracts were used for the phytochemical identification test and MTT assays using HepG2 and A375 cell lines.

Results:
The antiproliferative activity study showed that the calculated IC50 values for the individual O. stamineus and P. granatum extracts against the HepG2 cell line were (144.46 µg/mL and 174.16 µg/mL), respectively, and against the A375 cell line, the IC50 values were determined to be (136.19 and 152.77 µg/mL), respectively. For the combined O. stamineus and P. granatum extracts against the HepG2 cell line, the calculated IC50 values were (138.4 ± 0.86 µg/mL, 124.52 ± 1.62 µg/mL, and 157.88 ± 1.31 µg/mL) against HepG2 at the combination ratios of 1:1, 2:1, and 1:2, respectively, and (106.33 ± 1.14 µg/mL, 99.62 ± 0.64 µg/mL, and 118.32 ± 1.06 µg/mL) for the A375 cell line at the combination ratios of 1:1, 2:1 and 1:2, respectively.

Conclusion:
This study showed that it actually takes two to tango since the antiproliferative activities of the combined extracts against the studied cell lines were stronger than their individual activities. Therefore, it may be suggested that the phytochemicals responsible for the observed activity in this study can be isolated and characterized for improved exploitation.

Key words: Averrhoa bilimbi; Eurycoma longifolia; Morus nigra; Orthosiphon stamineus; Punica granatum.