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Introduction: Background Material
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
A reversible reaction proceeds in both the forward direction (reactants ⟶ products) and the reverse direction (products ⟶ reactants), resulting after a sufficiently long time in a state of dynamic equilibrium consisting of a mixture of reactants and products. For example, consider the simple, first-order reaction:
In Vitro Stability of Radiotracers
Published in Lelio G. Colombetti, Principles of Radiopharmacology, 1979
Leopoldo L. Camin, Eugene L. Saklad
But optimal treatment of the data is dependent upon the mode of degradation; in general, the order of the reaction. The basic techniques employ the familiar linearized representations of frequently nonlinear reaction kinetics. In the following example, the decomposition of pyrophosphate (as freeze-dried stannous pyrophosphate) would plot on a rectilinear grid as a family of temperature-dependent exponential curves. As indicated mathematically (Table 2), for a simple first order reaction there is a linear change in the logarithm of the measured species with time. There is a frequent temptation to normalize all stability data by expressing it as a fraction of its initial value, thereby hoping to compensate for differences in initial values from lot to lot. This convenient technique is applicable only for first order reactions, because the difference between the logarithms of any pair of values is equal to the logarithm of the quotient of the same two values, as shown in Table 2. (For reactions other than first order, the slopes would change for curves with different initial values, but would be constant for curves based upon differences between the values or the appropriate functions of the values.)
Targeted Molecular Imaging in Cardiology
Published in Robert J. Gropler, David K. Glover, Albert J. Sinusas, Heinrich Taegtmeyer, Cardiovascular Molecular Imaging, 2007
The practical approach to monitor these low density targets by external imaging came much later as gamma and positron emitting radionuclides and improved imaging devices became available and has been based on the bimolecular model, which for high specific activity radioligands, reduces to a first order reaction. The common radionuclides for single photon emission computed tomography (SPECT) are 99mTc (half-life = 6 h) and 123I (half-life = 13 h). Radionuclides for positron emission tomography (PET) emit a positron that annihilates to give two 511 keV gamma rays at approximately 180 degrees. The positron-emitting radionuclides (with their half-lives) used most frequently are: 15O (2.07 min), 11C (20.4 min), and 18F (109.7 min). The specific activities (Ci/mmol) of these radionuclides are high because they are made through a nuclear transformation; that is, one element is converted into another so that, except for trace contaminants, they are carrier free. The actual specific activities for the most-used PET radionuclides, 18F and 11C, are of the order of 1000–5000 Ci/mmol at the end of the cyclotron bombardment due to contamination by fluoride and carbon dioxide, respectively. Therefore, these radioactive probes are injected at tracer levels (~2–10 nmol injected) for a 10 mCi dose. The 123I radioisotope of iodine can be obtained at near the theoretical specific activity of 233,700 Ci per mmol and therefore involves the injection of ~50 pmol. The uniqueness of the nuclear medicine technique based on this tracer principle is in measuring biochemistry in vivo, especially the biochemistry of low density sites, such as receptors, by external imaging. PET emphasized neuro-chemistry in the first efforts in external imaging and therefore most of the early imaging probes for low density sites were targeted to neuroreceptors (Table 1).
Degradation mechanism of Direct Red 23 dye by advanced oxidation processes: a comparative study
Published in Toxin Reviews, 2022
Mohamed A. Hassaan, Ahmed El Nemr, Adel A. El-Zahhar, Abubakr M. Idris, Majed M. Alghamdi, Taher Sahlabji, Tarek O. Said
To study the kinetics of the degradation process, the pseudo-first-order kinetic model was applied as presented in Equation (2). The kinetic parameters were determined by plotting Ln(Ct/C0) against the reaction time. The half-life (t1/2) of the removal reaction was determined, which represents the time of degraded DR-23 dye to its half of the initial concentration. For the first-order reaction, the t1/2 was calculated using Equation (3); where Ct (mg/L) is the concentration of DR-23 dye at time t, C0 (mg/L) is the initial concentration of DR-23 dye, kobs (1/min) is the observed rate constant:
Ethylcellulose microparticles enhance 3,3′-diindolylmethane anti-hypernociceptive action in an animal model of acute inflammatory pain
Published in Journal of Microencapsulation, 2020
Juliane Mattiazzi, Marcel Henrique Marcondes Sari, Paulo Cesar Oliveira Araujo, Andrei Vinícius Englert, Jéssica Mendes Nadal, Paulo Vítor Farago, Cristina Wayne Nogueira, Letícia Cruz
Ethylcellulose MPs often present first-order kinetics as the best model to explain the release profile of the compound from the polymer matrix (Rogers and Wallick 2011, 2012a, 2012b). By adjusting the data of the present study to the first-order reaction kinetic equation, the value found for the rate constant k was 0.0116 h−1 with a correlation coefficient of 0.9313. These results indicate that the DIM release from the MPs occurs proportionally to the remaining compound concentration within the microstructure. The medium diffuses through the polymeric matrix and dissolves DIM, reducing its amount in the MPs, thus, the quantity released will exponentially decrease over time.
Drug-interactive mPEG-b-PLA-Phe(Boc) micelles enhance the tolerance and anti-tumor efficacy of docetaxel
Published in Drug Delivery, 2020
Feirong Gong, Rongrong Wang, Zhengquan Zhu, Jiayao Duan, Xin Teng, Zhong-Kai Cui
The pharmacokinetic profiles of DTX-PMs and Taxotere® after i.v. administration are shown in Figure 6 and the calculated pharmacokinetic parameters are listed in Table 2. Kel was the elimination rate constant for DTX according to the laws of the first-order reaction kinetics, t1/2 is the elimination half-life of DTX and is calculated using 0.693/Kel, tmax is the time to reach the maximum concentration (Cmax) and CL was the clearance of DTX from the body. The variation of the DTX concentration after i.v. administration of DTX-PMs was similar to that of Taxotere®, while the DTX level in the group treated with DTX-PMs was always higher than the latter, both in the whole blood (Figure 6(A)) and in the plasma (Figure 6(B)). The area under the curve (AUC) showed that DTX exposure of DTX-PMs was significantly higher than that of Taxotere® in plasma (p<.05). The apparent volume of distribution calculated using the steady-state method (Vdss) of DTX from DTX-PMs was evidently lower than that from Taxotere® in the whole blood (p<.05). Those results revealed that DTX administrated in the form of DTX-PMs partitioned slower from plasma to tissues compared with DTX delivered by Taxotere®, which might result from the improved stability through micelle encapsulation of the drug. Furthermore, Cmax and mean residence time (MRTIV) of DTX-PMs were significantly higher (p<.05) than the values of Taxotere® both in the whole blood and plasma. The half-life of DTX in the DTX-PMs was reduced slightly in the whole blood compared with that in Taxotere®.