Innovative Technologies: Organic Solar Cells

Photovoltaic cells that convert sunlight into electricity have been around for decades, yet their commercial use has been largely limited to applications where conventional electric power is difficult or impossible to provide, such as lighting of road signs and offshore buoys. The problem is primarily economic—although sunlight is free, the high cost of manufacturing traditional silicon-based solar cells has limited their penetration into markets where coal, nuclear, and other nonrenewable sources currently provide more economical energy. Researchers at the Georgia Institute of Technology have developed a new type of solar cell that may someday change that equation. 
 
Bernard Kippelen, a professor in the Center for Organic Photonics and Electronics and the School of Electrical and Computer Engineering at Georgia Tech, is leading studies into the use of pentacene as a medium for converting sunlight to electricity. Pentacene, a compound of carbon and hydrogen, can form a crystalline film in which molecules assemble in an ordered pattern. This makes the compound more conducive to the flow of electricity than the disordered organic compounds that have been tested in the past for possible photovoltaic applications. Improved conductivity leads to higher efficiency, and if that quality can be combined with low cost of manufacture and ease of use, the material holds great promise. 
 
In an article published in the 29 November 2004 issue of Applied Physics Letters, Kippelen and fellow research scientists Seunghyup Yoo and Benoit Domercq describe their tests of an organic film made of pentacene combined with a form of carbon known as C60. The organic layers and an electrode were sequentially deposited onto indium–tin oxide substrates. Broadband illumination was provided by a lamp, photocurrent was measured under varying light spectrums, and conversion efficiencies (the amount of light converted into electricity) were calculated. 
 
The team was able to convert solar energy into electricity with 2.7% efficiency; in unpublished tests since then, they demonstrated power conversion efficiencies of 3.4%. Kippelen believes they will be able to reach 5% in the near future. 
 
Commercial photovoltaic cells that employ silicon crystals are 12–15% efficient, but they are expensive to manufacture and run. Complete systems, including installation, produce electricity at a cost equivalent of 20–40¢ per kilowatt hour (depending upon scale of system and financing) versus 8–12¢ per kilowatt hour for electricity generated by conventional power plants. Kippelen says development of thin-film organic solar cells is not far enough along to estimate the costs of energy production. However, the thin-film cells could possibly be manufactured in a roll-to-toll process, significantly lowering their cost and narrowing the gap with fossil fuel–generated electricity. 
 
Kippelen is confident that his product’s unique properties will allow it to be used in applications for which silicon cells are not appropriate. Whereas silicon cells are rigid and relatively thick at 100 microns across, thin-film organic solar cells are lightweight, flexible, and less than 1 micron thick. This could open up new markets for solar energy, perhaps powering small electronic devices such as radiofrequency identification tags, MP3 players, and laptop computers. Kippelen estimates that organic solar cells are at least five years away from residential applications but could find niche low-power applications within two years. 
 
However, thin-film solar power will need to be deployed on a much larger scale if it is to significantly improve the environment. “Small electronic devices represent a miniscule part of total energy consumption,” says Tom Starrs, chairman of the American Solar Energy Society. “For any photovoltaic technology to make a significant contribution to global energy needs, it needs to be interconnected with the electrical grid, displacing power generated by coal, nuclear, and other nonrenewable sources of energy.”


Background
Hydromorphone hydrochloride (HCl), which is available in immediate-and extended-release formulations, is a semi-synthetic opioid agonist that has been used widely for many years in the treatment of acute and chronic pain.
A number of studies have demonstrated the efficacy and tolerability of hydromorphone in comparison with morphine and other opioid analgesic agents [1]. When formu-lated as an immediate-release preparation, hydromorphone has an elimination half-life of approximately 2 to 3 hours [2][3][4]. As a consequence, doses must be administered every 4 to 6 hours to ensure continuous analgesia for the patient [5].
To improve pain relief and provide convenient dosing for patients with severe chronic cancer and non-cancer pain, a novel 24-hour controlled-release formulation of hydromorphone is currently being investigated. This formulation uses the patented OROS ® Push-Pull™ osmotic pump delivery system developed by ALZA Corporation (Palo Alto, CA) [6][7][8], and a consistent release of hydromorphone over 24 hours has been demonstrated in healthy volunteers [9]. Moreover, steady-state plasma concentrations for OROS ® hydromorphone (Jurnista™, Janssen Pharmaceutica, N.V., Beerse, Belgium) are achieved after 48 hours (i.e., after two doses or by the third dose) and are maintained throughout the 24-hour dosing interval [10]. An initial study also has shown that the pharmacokinetics of hydromorphone are not substantially affected when OROS ® hydromorphone is taken immediately after a high-fat meal [11].
Co-administration of OROS ® hydromorphone with naltrexone, an opioid antagonist, under fasting conditions resulted in a 39% increase in C max , but there was no significant change in T max , AUC 0-t , or AUC 0-∞ [11]. These results indicate that blockade of opioid effects by naltrexone is useful in comparative bioavailability studies of high-dose opioids in healthy volunteers, with the assumption that all treatments are affected similarly. The objective of the present study was to evaluate the dose proportionality and linearity of OROS ® hydromorphone at daily doses of 8, 16, 32, and 64 mg.

Subjects
Study volunteers were non-smoking, healthy male and female adults between 19 and 50 years of age. Their body weight was required to be between 61 and 100 kg and within ± 10% of the recommended weight range for height and body frame (1984 Metropolitan Height and Weight Tables). Results of the baseline screen were required to be negative for drugs of abuse (cannabinoids, opiates, cocaine, ethanol, and barbiturates). Subjects were required to have no clinically significant deviations from normal in laboratory results. All participants provided written informed consent. The study was approved by the Institutional Review Board and was carried out according to the Declaration of Helsinki and subsequent revisions.
Subjects who were intolerant of, or hypersensitive to, opioid agonists or antagonists were excluded, as were those with opioid dependency. Other exclusion criteria included gastrointestinal disorders; compromised cardiac, respiratory, renal, or hepatic function; psychiatric abnormalities; and significant hematologic, metabolic, or central nervous system disorders. Study participation did not permit any subject to take any long-term medication, enzyme-altering agents, recreational drugs, or an investigational agent within 30 days of beginning the study.

Study design and interventions
This was an open-label, randomized, four-way crossover study designed to examine the pharmacokinetic profile of once-daily OROS ® hydromorphone for dose proportionality after administration of a single oral dose of 8, 16, 32, and 64 mg.
Based on the assumption that the within-subject variability is less than 20% (value guided by variability in exposure following immediate-release hydromorphone) and that there is a 5% difference between treatments, a sample size of 30 subjects was estimated to provide 80% power to demonstrate equivalence at the 0.05 level of significance.
Subjects received each of the four treatments (OROS ® hydromorphone 8, 16, 32, and 64 mg, given after a 10hour overnight fast), with a 7-day washout period between treatments. The order in which treatments were received was determined according to the predetermined randomization schedule. Naltrexone 50 mg was administered 12 hours before, with, and 12 hours after OROS ® hydromorphone in all groups, with an additional 50-mg dose of naltrexone administered 24 hours after the 64-mg dose of OROS ® hydromorphone. Naltrexone was administered to minimize adverse events following the higher doses of OROS ® hydromorphone in these opioid-naïve subjects, and was given concomitantly with each dose level of OROS ® hydromorphone to facilitate dose-proportionality comparisons.

Plasma sampling
Plasma samples for pharmacokinetic analysis were collected pre-dose (time 0) and at 2, 4, 6, 8, 10, 12, 16, 20, 24, 30, 36, 42, and 48 hours post-dose. Additional samples were taken at 56, 64 and 72 hours after the 64-mg dose. Plasma hydromorphone concentrations were measured using a validated LC/MS/MS method (CEDRA Corporation, Austin, TX) covering a range of 0.05 to 10 ng/ mL. Calibration standards prepared for each of the sample sets were used to calculate the inter-day precision of the assay. The coefficients of variation for the standards ranged from 1.7% to 9.9%. The absolute deviations ranged from 0.05% to 2.6%.
Based on the measured hydromorphone concentration, the following parameters were calculated: peak plasma concentration (C max ), time at which peak plasma concen-tration was observed (T max ), terminal half-life (t 1/2 ), and the area under the concentration-time curve from time 0 to time t (AUC 0-t ) and from time zero to infinity (AUC 0-∞ ). The non-compartmental pharmacokinetic parameters described above were estimated using macros built in Excel (Microsoft, Redmond, WA).

Statistical analysis
Untransformed and log-transformed (ln) data for C max , AUC 0-t and AUC 0-∞ were analyzed using an appropriate analysis of variance (ANOVA) regression model to establish dose linearity and dose proportionality. All tests were two-sided at the 0.05 level of significance. T max was analyzed non-parametrically, without dose-normalization, using the Wilcoxon matched-pairs test for each pairwise comparison; the 95% confidence interval (CI) for the difference in treatment medians was constructed. Data for t 1/ 2 were summarized using descriptive statistics. The apparent elimination-rate constant (K) for each subject was estimated by linear regression of the log-transformed concentration during the terminal log-linear decline phase of the curve. Terminal half-life was estimated as 0.693/K.

Subjects
Thirty-two healthy volunteers were enrolled in the study, 8 in each of four treatments, with at least 24 subjects expected to complete the study. They were primarily male (63%) and Caucasian (81%), with a mean age of 33 years ( Table 1). The study was completed by 31 subjects; one subject discontinued for personal reasons, after completing the first phase of treatment (64-mg dose).

Pharmacokinetics
The plasma concentration-time profiles of the four OROS ® hydromorphone doses tested are shown in Figure  1. Following a single oral dose of OROS ® hydromorphone, plasma mean concentrations gradually increase over 6 to 8 hours, and thereafter are sustained at or near maximum levels up to approximately 30 hours post-dose. The means of untransformed pharmacokinetic parameters and the medians of T max are shown in Table 2. Maximum plasma hydromorphone concentrations were achieved approximately 12 to 16 hours after administration, with no significant dose effect observed. Mean values for t 1/2 were similar for the various doses (10.6-11.0 hours). Analysis of C max , AUC 0-t , and AUC 0-∞ by dose indicated that the relationship was linear (P ≤ 0.05) and that the intercept did not differ significantly from zero (P > 0.05; Figure 2).
Mean dose-normalized pharmacokinetic parameters for OROS® hydromorphone after administration of 8, 16, 32, and 64 mg doses are shown in Table 3. Cmax and AUC increased linearly and in a manner proportional to the dose of OROS® hydromorphone. The slopes of dose-normalized Cmax and AUC vs. dose did not differ significantly from zero (P > 0.05; Figure 3). Inter-subject variability in pharmacokinetic parameters was similar across the doses except for high variability of Cmax following the 8-mg dose. This was mainly due to one subject with a high concentration (>5 times the mean). When this subject was excluded, Cmax variability for the 8-mg dose was similar to the other doses. No significant gender-bytreatment interactions were observed (ANOVA model; data not shown).

Safety
At least one adverse event was experienced by 21 of the 32 subjects (66%). All events were of mild or moderate intensity, and none were considered serious. Headache, asthenia, and nausea were the most common adverse events, occurring in 31%, 28%, and 28% of patients, respectively, during one or more of the treatment periods. The adverse events for each dose group are shown in Table 4. No treatment-related trends were noted with regard to vital signs, electrocardiogram results, or clinical laboratory data.

Discussion
The results of this study indicate that plasma hydromorphone concentrations and overall exposure to hydromorphone are proportional to the administered dose (over the 8-to 64-mg dose range) with OROS ® hydromorphone. The time to achieve maximum plasma concentration was independent of dose. Near-maximum plasma concentrations were reached approximately 6 hours after dosing, and plasma concentrations were maintained at or near maximum levels throughout a 30-hour period, consistent with once-daily dosing. Beginning 24 to 30 hours postdose, plasma hydromorphone concentrations declined slowly, with an apparent terminal half-life of approximately 10 hours. This is longer than the half-life of immediate-release hydromorphone (2-3 hours), which has been determined from studies with intravenous formulations [2][3][4]. The present study included plasma sampling for up to 72 hours post-dose, and it was designed to characterize both the controlled-release and the post-absorptive elimination phases of the drug. The apparent terminal half-life observed in this study is similar to that seen in a study designed to assess the effects of food intake on the pharmacokinetics of OROS ® hydromorphone [11]. The observed plasma profile with concentration maintained over 24 hours supports the proposed once-daily administration of OROS ® hydromorphone.
An exploratory analysis suggested no influence of gender on the pharmacokinetics of OROS ® hydromorphone for the dose range studied. Although limited, these data do suggest that there are no clinically relevant differences Mean plasma hydromorphone concentrations over time after administration of single-dose OROS ® hydromorphone Figure 1 Mean plasma hydromorphone concentrations over time after administration of single-dose OROS ® hydromorphone.  between males and females with respect to the pharmacokinetics of OROS ® hydromorphone.
Safety results were consistent for all four OROS ® hydromorphone doses, indicating no dose relationship with the incidence of adverse events. Adverse events were consistent with those expected for an opioid agonist and antagonist and primarily affected the digestive and central nervous systems. No serious adverse events were reported during the study.

Conclusion
Plasma concentrations of OROS ® hydromorphone and its pharmacokinetic parameters were found to be proportional to the orally administered dose over the dose range studied (8 mg to 64 mg). Plasma concentrations achieved the maximal level by approximately 16 hours after single administration, independently of dose, and remained near that level for up to 30 hours. Adverse events were consistent with those expected for an opioid agonist and antagonist.