## 1 Introduction  
Pitaya or dragon fruit (_Hylocereus_ spp.) is a climbing vine cactus species which has successfully attained international recognition, both as an ornamental plant and as an economical fruit crop. Its fruit is regarded as the most beautiful one in the _Cactaceae_ family, with a bright red skin studded with green scales and white or red flesh with well dispersed small black seeds. Pitaya originates from the tropical forest regions of Mexico and Central and South America [1]. There are three varieties of pitaya; namely white flesh pitaya with a yellow peel (_Selenicereus megalathus_), white flesh pitaya with a red peel (_Hylocereus undatus_) and red flesh pitaya with a red peel (_Hylocereus polyrhizus_) [2; 3]. Red pitaya (_Hylocereus polyrhizus_) is being extensively cultivated in Malaysia, Thailand, Vietnam, Australia, Taiwan, and some other parts of the world. It is increasingly gaining attention in many countries due to its exotic aesthetic characteristics, high economic potential, and exceptional tolerance to extreme drought [4]. Reactive oxygen species (ROS) are among the most significant factors that intensively contributes to the ultraviolet (UV) radiation induced skin pigmentation [5]. Keratinocytes adjacent to melanocytes produces nitric oxide radicals (NO\({}^{\circ}\)) that triggers the increment of the level of melanogenesis factors, namely, tyrosinase and tyrosinase-related protein-1 (TRY-1) [6]. This phenomenon increases the rate of melanogenesis which results in an abnormal production of skin pigments that causes the darkening of the skin that is aesthetically unfavorable. Therefore, several studies have emphasized the importance of phytochemicals with good antioxidant potentials in the regulation of skin pigmentation to curb the effects of ROS. Vitamin C is undoubtedly one of the highly utilized antioxidants for skin depigmentation due to its efficiency in neutralizing the ROS through the radical scavenging process and also for its exceptional tyrosinase enzyme inhibiting properties [7]. Despite the high oral dosage, an active transport mechanism restricts the absorption of vitamin C in the gut and eventually only a limited amount of the drug is absorbed into the system [8]. Thus, the inadequacy of the bioavailability of vitamin C through oral administration encourages the use of topical ascorbic acid in the field of dermatology [9]. If the active ingredients consist of therapeutic values or prophylactic in human subjects, then it is vital to perform a proper extraction to retrieve them [10]. Besides that, an appropriate extraction condition ensures that the valuable endogenous bioactive components are not damaged and the functional potential of the raw materials is well preserved throughout the process [11]. Thus, the optimization of extraction conditions of the red pitaya peels plays an essential role to maximize its antioxidant potentials. More importantly, oxidative stress caused by ROS under antioxidant deprivation is one of the crucial mechanisms that stimulate skin darkening and other dermatological disorders. Therefore, optimizing the antioxidant potentials of the red pitaya peel extract is quintessential in treating or preventing those conditions [12]. Response Surface Methodology (RSM) is a powerful statistical tool with high precision which is widely employed for the optimization of complex processes and extraction technologies as it depicts the complete effects of the variables, evaluates the interactions between multiple parameters, reduces the number of experimental trials, and shortens the trial time. Phytochemicals isolated from plants possess the ability to provide a substantially unexplored alternative for the invention of new drugs to be utilized in the cosmeceutical and pharmaceutical industries. The incorporation of plant-based bioactive compounds for the formulation of current skin care products is continually being highlighted due to the possible side-effects that might be posed by the usage of synthetic active ingredients.