Chemical composition and mechanism of action

In certain cases, cholesterol is required for vesicle formation. It is commonly accepted that the hydrophilic lipophilic balance (HLB) is a parameter that could indicate the vesicleforming potential of surfactants. For amphiphils such as sorbitan esters and alkyl ethers, low HLB values could predict vesicle formation [52,55]. However, niosomes were obtained from polysorbate 20 (HLB 16.7), a highly hydrophilic molecule, when cholesterol at an appropriate concentration was added to the amphiphil [44], In this case it could be assumed that a kind of amphiphilic complex with a lower HLB was responsible for the vesicle formation. An excellent review on the structure, characteristics, chemical composition, and mechanism of action of niosomes was published by Uchegbu and Vyas [41]. 

Usually aquatic toxicity of chemicals with general narcosis mechanism of action is described by the octanol/water partition coefficient [73]. However, log is a composite descriptor which has components of molecular volume and H-bond acceptor terms. Raevsky and Dearden [74] therefore used molecular polarizabihty (as a volume-related term) and the H-bond acceptor factor instead of log to model aquatic toxicity (log LC50) to the guppy for 90 chemicals with general narcosis mechanisms. This excellent correlation has statistical criteria better than that obtained for the same data using log Pofy, ... 

The prediction of the toxicological properties of a chemical mixture requires detailed information on the composition of the mixture and the mechanism of action of each of the individual compounds. In order to perform a risk assessment, proper exposure data are also needed. Most often... 

LPS immunotherapy was the first immunotherapy for cancers assayed in patients in spite of its toxicity. The standardisation of animal models of cancer, the discovery of the LPS composition and of lipid A activity, the discovery of lipid A structure leading to its chemical synthesis, and the synthesis of lipid A derivatives far less toxic than the natural lipids A, restarted research in this field. At the same time, advances in immunology allowed a better understanding of the mechanisms of action of LPS and lipids A in whole organisms. 

The stabilizers chosen for evaluation include different types of heat and light stabilizers selected to represent different mechanisms of action as well as chemical compositions (ArJi). Types of stabilizers evaluated include benzotriazole and benzophenone light stabilizers [ultraviolet (UV) light absorbers], hindered amine light stabilizers (HALS, catalytic radical scavengers), hindered phenol heat stabilizers (antioxidant radical scavengers), and thioester heat stabilizers (antioxidant hydroperoxide decomposers). 

Many compounds can act as adjuvants. Their classification is made difficult by the variety in chemical composition and the overlapping, often poorly understood, mechanisms of action. Only aluminum adjuvants are approved by the FDA for use in human vaccines. Quil A is a saponin that is commonly used as an adjuvant in veterinary vaccines and is also a component of immune-stimulating complexes (ISCOM). These adjuvants are addressed in some detail below. A detailed discussion of other types of adjuvants can be found in recent books and reviews on this subject. 

The pharmacokinetics of wood creosote, coal tar creosote, coal tar, coal tar pitch, and coal tar pitch volatiles have not been defined because of their chemical complexity. Creosotes vary tremendously in composition and hence, mechanisms of action most likely differ among individual samples of creosotes. Information on individual components is not adequate to define the properties of the whole mixture and for this reason no PBPK models have been proposed for creosote. 

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