Real-time electrochemical monitoring of drug release from therapeutic nanoparticlesIntroductionMaterials and methodsReagents and apparatusProcedureResults and discussionOptimization and analytical performanceVoltammetric monitoring of doxorubicin release from liposomes in PBS bufferVoltammetric monitoring of doxorubicin release from liposomes in human serumGeneral commentsConclusionsAcknowledgementsSupplementary dataReferencesReal-time electrochemical monitoring of drug release from therapeutic nanoparticlesLaura Moraa,c, Karin Y. Chumbimuni-Torresa, Corbin Clawsonb, Lucas Hernandezc,Liangfang Zhanga,⁎, Joseph Wanga,⁎aDepartment of Nanoengineering, University of California San Diego, La Jolla, CA 92093, USAbDepartment of Bioengineering, University of California San Diego, La Jolla, CA 92093, USAcDepartment of Analytical Chemistry, Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, Spainabstractarticle infoArticle history:Received 8 May 2009Accepted 2 August 2009Available online 11 August 2009Keywords:NanomedicineDrug releaseReal-time monitoringVoltammetryLiposomeNanoparticlesDoxorubicinAn electrochemical protocol for real-time monitoring of drug release kinetics from therapeutic nanoparticles(NPs) is described. The method is illustrated for repetitive square-wave voltammetric measurements of thereduction of doxorubicin released from liposomes at a glassy-carbon electrode. Such operation couples highsensitivity down to 20 nM doxorubicin with high speed and stability. It can thus monitor in real time thedrug release from NP carriers, including continuous measurements in diluted serum. Such direct andcontinuous monitoring of the drug release kinetics from therapeutic NPs holds great promise for designingnew drug delivery NPs with optimal drug release properties. These NPs can potentially be used to delivermany novel compounds such as marine-life derived drugs and hydrophobic drugs with limited watersolubility that are usually difficult to be characterized by traditional analytical tools.© 2009 Elsevier B.V. All rights reserved.1. IntroductionNanoparticle (NP)-based drug delivery has attracted tremendousattention from both academic and industrial investigators in the pasttwo decades because of its many favorable characteristics [1,2].Itimproves the solubility of poorly water-soluble drugs, prolongs in vivodrug circulation half-life, reduces the frequency of administration byreleasing drugs in a sustained manner, and minimizes adversesystemic effects by delivering drugs preferentially to the targettissues. As a result, numerous NP platforms have been developed orproposed for drug delivery applications incl uding, for example,liposomes, solid lipid NPs, polymeric NPs, dendrimers, silica NPs,and nanoemulsions [3,4]. For all of these therapeutic NPs, their drugrelease kinetics is a key factor that determines their therapeutic indexand potential for clinical use [2,5]. Drug release kinetics representshow fast the drug molecules are released from the therapeutic NPs.Such a release profile is commonly plotted as the weight ratio of thecumulative released drugs to the total drug payload over time [6].Direct real-time measurements are highly desirable for obtainingreliable assessment of the drug release kinetics.While several analytical techniques have been employed toquantify the amount of drugs released from therapeutic NPs [7–10],none of these offers a direct continuous monitoring capability. Highperformance liquid chromatography (HPLC) holds the most popular-ity in quantifying drugs by directly reading their characteristic UVabsorbance upon elution from a proper HPLC column [11]. HPLC thuslacks the real-time monitoring capability and suffers from highprocurement and operation al costs, lengthy traini ng required ,excessive downtimes, and lack of a universal sensitive detector [12].A fluorometer or scintillation counter can also quantify drugs byreading the fluorescence emission or the ionizing radiation of drugmolecules, respectively, if they are pre-labeled with a fluorescent orradioisotope tag. However, tagging drug molecules may alter theirdiffusion rate and release kinetics.To measure drug release kinetics, therapeutic NPs are commonlyloaded into a dialysis device with a molecular weight cut-off largerthan the size of the drug molecules [6,13]. Then the NPs are dialyzedagainst PBS buffer continuously. The released drugs diffuse out of thedialysis device, driven by osmotic pressure between the two sides ofthe dialysis membrane. At selected time intervals, a small volume ofthe dialysis solution is collected to quantify the drugs released fromthe NPs using one of the techniques mentioned above. Alternatively,one can also collect an aliquot of the NP suspension inside the dialysisdevice and subsequently break down the NPs to quantify the drugsremaining in the NPs [13]. While these techniques are capable ofquantifying drug loading yield and release profile, they usuallyinvolve complex procedures and require labor-intensive samp lepreparation. In addition, before measurements the drugs have to beseparated from the NPs by using a dialysis membrane or a centrifugalmachine. These can negatively affect the accuracy of the measure-ments because drugs can absorb to the dialysis membranes or thehigh centrifugal force may induce additional drug release from theJournal of Cont rolled Release 140 (2009) 69–73⁎ Corresponding authors.E-mail addresses: [email protected] (L. Zhang), [email protected] (J. Wang).0168-3659/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jconrel.2009.08.002Contents lists available at ScienceDirectJournal of Controlled Releasejournal homepage: www.elsevier.com/locate/jconrelNANOMEDICINENPs [14]. More importantly, none of these techniques can monitordirectly the real-time drug release, as desired for obtaining the mostaccurate drug release kinetics profile. The sample preparation time forthe various techniques also prohibits repetitive measurements atshort time intervals. Therefore, it is highly desirable to develop newmethods that monitor directly and continuously the drug releasekinetics from therapeutic NPs while concurrently simplifying theprocess and minimizing the errors and costs incurred in the collectionand handling of individual sample aliquots.In this study, we describe an effective electrochemical protocol fordirect real-time monitoring of drug release kinetics from therapeuticNPs. Electrochemical devices