Colloids and Surfaces B: Biointerfaces 134 (2015) 8–16 Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces jo ur nal ho me p ag e: www.elsevier.com/locate/colsurfb Physical factors affecting chloroquine binding to melanin R.L. Schroeder, P. Pendleton, J.P. Gerber ∗ School of Pharmacy and Medical Sciences, University of South Australia, North Terrace, Adelaide 5000, Australia a r t i c l e i n f o Article history: Received 26 February 2015 Received in revised form 16 June 2015 Accepted 17 June 2015 Available online 25 June 2015 Keywords: Melanin Sepia Chloroquine Adsorption Isosteric heat pH a b s t r a c t Chloroquine is an antimalarial drug but is also prescribed for conditions such as rheumatoid arthri- tis. Long-term users risk toxic side effects, including retinopathy, thought to be caused by chloroquine accumulation on ocular melanin. Although the binding potential of chloroquine to melanin has been investigated previously, our study is the first to demonstrate clear links between chloroquine adsorption by melanin and system factors including temperature, pH, melanin type, and particle size. In the cur- rent work, two Sepia melanins were compared with bovine eye as a representative mammalian melanin. Increasing the surface anionic character due to a pH change from 4.7 to 7.4 increased each melanin’s affin- ity for chloroquine. Although the chloroquine isotherms exhibited an apparently strong interaction with each melanin, isosteric heat analysis indicated a competitive interaction. Buffer solution cations com- peted effectively at low surface coverage; chloroquine adsorption occurs via buffer cation displacement and is promoted by temperature-influenced secondary structure swelling. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The association between ingested drugs and melanin in the body has been the subject of many studies over the last 50 years. For example, melanin has a strong affinity towards compounds such as phenothiazines [1–6], antibiotics [1], antimalarials [2–6], antirheumatics [2], antifolates [1], as well as various illicit drugs [7–9] and herbicides [10], with each causing various adverse effects. Due to these interactions, various negative associations have been implicated with some compounds, resulting in outcomes unrelated to the main action of the drug. Chloroquine has been used widely for the treatment of malaria and other conditions, including rheuma- toid arthritis, discoid lupus erythematosus and amoebic hepatitis, and has also been associated with delayed-onset retinopathy [11,12]. The present study focused on gaining a greater insight into the thermodynamics and mechanism of the melanin–chloroquine interaction. It is anticipated such knowledge would be connected to the associated pathologies in future research. The accumulation of a particular drug and its mode of action ultimately determines its toxicity [13]. An irreversibly bound xenobiotic will not necessarily cause adverse effects, but rather depends on tissue or organ specificity. If no harm arises from ∗ Corresponding author. Tel.: +61 8 8302 2568; fax: +61 8 8302 1087. E-mail addresses: rhiannon.schroeder@mymail.unisa.edu.au (R.L. Schroeder), Phillip.Pendleton@unisa.edu.au (P. Pendleton), Cobus.Gerber@unisa.edu.au (J.P. Gerber). the interaction, a substance such as melanin would simply act as a deactivating reservoir. However, the injury arises when accu- mulation (as localised adsorption) occurs in regions of the body where the released drug could cause damage [13]. The corre- lation between the percentage of tissue accumulation and the amount of pigment in the eye suggests a relationship may exist between retinopathy and melanin. It was proposed that retinopa- thy caused by chloroquine was either due to its adsorption to ocular melanin [14] or due to another pathophysiological pathway caused by chloroquine itself [15]. A third hypothesis suggested that chloroquine chemisorbs with melanin possibly leading to retinal toxicity through an alteration of the melanin surface, and sub- sequently undermining the protective role of melanin as a free radical scavenger [16]. Either way, melanin is perceived to play a role. The structural analogue of chloroquine, hydroxychloroquine, is less toxic and has not been linked to retinopathy. We have shown that this reduced toxicity might be interpreted in terms of its adsorption mechanism. Hydroxychloroquine adsorbs through weaker interactions than chloroquine to different types of melanin [17], suggesting that adsorption to melanin may be a deciding factor in determining toxicity. Organic melanin production is highly regulated in melanosomes. The melanogenetic pathway begins with the hydroxylation of tyrosine to dopa through the action of the rate- limiting enzyme tyrosinase. Dopa progresses through a series of reactions from the oxidation to dopaquinone, which then cyclises to dopachrome. At this point, the intracellular conditions dictate the type of melanin produced. Dihydroxyindole (DHI) is formed http://dx.doi.org/10.1016/j.colsurfb.2015.06.040 0927-7765/© 2015 Elsevier B.V. All rights reserved.