**1. Introduction**

Polyphenols, particularly flavonoids, have been widely studied for their bioactive properties and their strong antioxidant e ffects are well noted [1–3]. In addition, some plant antioxidant polyphenols exhibit anti-neoplastic activity by multiple pathways, including their potential to intensify the action

of cytostatic drugs, through the attenuating the multidrug resistance (MDR) phenomenon [4]. The best recognized and the most frequent cause of the MDR involves an increased activity of ATP-binding cassette family transporters (ABC) [5]. A large number of compounds have been identified as MDR suppressors by interfering with the P-gp-mediated export of chemotherapeutic agents [6–9]. However, in many cases the high toxicity of these substances has limited their use [10]. Thus, natural antioxidants have been identified as novel potential candidates [11–16].

Di fferent studies show that plants of the genus *Ageratina* contain a variety of flavonoids. Particularly, in our previous research studying the extracts obtained from the leaves of *Ageratina havanensis* (Kunth) R. M. King & H. Robinson, growing in Cuba, by chromatographic, spectroscopic and spectrometric methods, we identified a significant presence of flavonoids and their glucosides [17,18]. We also determined that the qualitative composition of the flavonoids in the plant is similar in two di fferent phenological stages, that is flowering and vegetative state [18].

Taking into account the abundance of flavonoids in the extracts prepared from *Ageratina havanensis,* it is expected that these extracts have antioxidant properties [19–21]. As qualitative composition of flavonoids is similar in both phenological stages, it could be hypothesized that the biological activity in both stages is similar, too. Based on these considerations, in this paper we studied the *Ageratina havanensis* (Kunth) R. M. King & H. Robinson extracts to prove their ability to inhibit P-gp function under no cytotoxicity conditions, their antioxidant potential and the influence of their quantitative composition on the biological properties of the plant.
