With the remarkable developments in the areas of separation science, spectroscopic. With the remarkable developments in the areas of separation science, spectroscopic techniques, microplate-based ultrasensitive in vitro assays and high-throughput screening HTS technologies, natural products research has gained momentum in recent years.
The pre-isolation analyses of crude extracts or fraction from different natural matrices, isolation, online detection and dereplication of natural products, studies on chemotaxonomy and biosynthesis, chemical finger-printing, quality control of herbal products, and metabolomic studies have now become much easier than ever before because of the availability of a number of modern sophisticated hyphenated techniques, e. This introductory chapter presents a general overview of the processes involved in natural products research, starting from extraction and isolation to elucidation of the structures of purified natural products and their bioactivity.
Isolation, Extraction, Bioassay, Natural products, Secondary metabolite. Antibody Data Search Beta. Authors: Satyajit D.
Sarker 1 ,. Lutfun Nahar 2. Combining of different, sometimes contrasting features has become a domain of the 21st century. The idea behind the hybrid is to make a new entity of not fully defined status joining independent elements or compounds which seem to be incompatible. The hybrid strategy in drug design is an alternative to co-formulating of two or more active substances in one tablet and to the classical cocktail therapy based on the administration of a few active substances in separate tablets.
This strategy aims to create new pharmacological activity with far more favorable biological effectiveness. This design approach in the search for bioactive molecules is a result of the molecular hybridization of two or more chemically active structures [ 6 , 9 ]. The word conjugated comes from the Latin word conjugare , which means mix or match. Chemical conjugation, which is of interest to rational medicinal chemistry, involves using the chemical reactivity of selected biologically active individuals and also drug- or non-drug linkers or carriers.
Codrug also known as a drug—drug conjugate or mutual prodrug consists of two or more drug compounds linked to one another via a labile covalent bond, in contrast to inactive prodrugs which are pharmacologically inactive medications that are metabolized into an active form within the body. It can be observed that the distinction between conjugate, codrug or hybrid type entities mentioned is very fluid and the molecular consortia concept are used in general to express and describe the new combined structures.
All the terms conjugate, codrug, hybrid etc. Incomplete understanding and lack of clear definition of these terms gives rise to confusion in the increasingly numerous studies concerning the design, synthesis, and biological properties of complex chemical species. Therefore, this paper describes attempts to classify and characterize one of the new dominant directions in the design of potential active structures—molecular consortia—for use mainly in the pharmaceutical field.
Many authors have tried to classify the synthesized forms, but there is no clear division and there are many convertible synonyms. Table 1 presents one of the most common classifications of compounds of this type. By increasing the number of pharmacophores subjected to chemical integration of chemical frameworks, trimeric and higher extended combinations are known in the literature as multitarget drugs or multiple ligands [ 9 , 10 , 11 ].
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The most popular and the most frequently reported framework combinations are dual connections, which are completely non-specific structural entities that open up completely new therapeutic perspectives. This paper mainly focuses on combinations of two forms. This strategy mentioned above leads to various novel combined compounds whose variety of nomenclature in references is different Table 2. Twin and triple drugs are defined as compounds that contain two and three pharmacophore components, respectively, exerting pharmacological effects in a one molecule. A twin drug bearing the same pharmacophores is a symmetrical twin drug homodimer , whereas the one possessing different pharmacophores is a non-symmetrical twin drug heterodimer [ 10 , 12 ].
In a triple drug the two identical portions could bind the same receptor sites simultaneously, while the third portion could bind to a different receptor site or enzyme [ 12 ]. In the dual structures showed in Table 1 , the main goal of framework combinations is to design two selected separate active molecules in one skeleton.
A possible combination of active compounds can be produced by an indirect or direct method, and depending on degree of frameworks integrated, can be viewed as: linked linked mode , fused no-linker mode and merged overlap mode [ 10 , 11 , 12 , 13 ]. The general scheme of options for drug-drug or drug-non-drug combinations is given as a multi-factor form called a molecular matrix, consisting of four factors: association mode, connection method, and the number of elements and linkers Figure 1.
The matrix presented here is a box with blocks, which contains various single elements that can be combined in different ways. The number of possibilities is determined by the availability of various elements and connectors appearing in the box set molecular matrix. In designing new compounds, we use the factors included in the proposed matrix and consider the following possibilities of connecting them. Association mode—describes a type of structural connection, interrelation between different selected elements, kind of attachment direct or indirect or structural cross-relationship linked, fused, merged.
Connection method—selected chemical and pharmaceutical units, depending on the functionality and reactivity, can form linear, pseudolinear or branched, smaller or larger, new structures.
Elements number—the most popular are structures built from two elements units, puzzles, segments but there can be many more of them. Linkers number—the number of intermediaries between selected elements in the structural population. Examples of various structural combinations are given in the Figures in the text. They illustrate different combinations of both small-small as well as large-small molecules and different types of connections: direct or indirect linker mode and merged ones, in the typical hybrid compounds.
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Selected components are integrated by a properly designed distinct linker group between two compounds forming a conjugate system. The choice of the right linker and the right method of attachment is the crucial part of conjugate designing [ 11 ].
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Linkers are classified to different categories according to the mechanism of drug release and drug stability in blood circulation system, including:. Cleavable linkers rely on the physiological environment of the body metabolism concentration of various agents, pH, presence of enzymes and disintegrate into two molecules which could then operate independently at separate biological targets.
Degradable linkers are often based on the ester linkage, which is easily hydrolysable by an esterase present in the plasma. Chain alkyl or aryl fragments, which may contribute to the additional or increased activity of the whole molecule are used as stable linkers. The linker chemistry determines the release profile of the active ingredients, and thus is crucial to the safety and efficacy of conjugates.
Moreover, both types of conjugates stable and cleavage are sometimes referred to as intermediate or linked hybrids [ 11 , 14 ]. Linkers are usually small, at least bifunctional reactive molecules, necessary for the chemical integration of different active substances. Any linkers used should allow attachment of the drug without causing inactivation of the bonded parts.
Linkers are divided into:. The most common functional groups are primary amines, sulfhydryls thiols , acids, alcohols, or bromides. The linker group can be:. However, in some different cases, the resulting hybrids, which are cleavage conjugates, can act as mutual prodrugs disintegrating on individual components [ 15 , 16 ]. Therefore, it is important to design a suitable—stable or unstable—linker that defines the pharmacological action.
Interesting properties, particularly in terms of the safety of use of nonsteroidal anti-inflammatory drugs NSAIDs , can be seen in the aspirin molecule ASA , which is modified by donors of nitric oxide. Aspirin-NO-donors hybrid structures are well tolerated and are more effective in inhibiting tumor cell growth than the standard molecule of aspirin, and even eliminate some side effects [ 10 ] Figure 2.
The other organic hybrid compounds with anti-inflammatory properties have been described extensively in the review by Mohsin and Ahmad [ 17 ]. The oxazolidine-quinolone derivative is promising drug substances for the treatment of infections, exhibiting both antibacterial and against tuberculosis activity [ 18 ] Figure 3.
Combining tacrine with a suitably substituted phenyl ring by means of an alkyloamine linker results in heterodimer-type compounds [ 14 ] Figure 4. The second approach to connecting frameworks involves direct combination of molecules fusion without the use of a linker [ 10 , 11 ]. The pharmacophores of both components do not overlap. Directly fused compounds have the size of the linker decreased such that the frameworks of the pharmacophores are essentially touching, which can be perfectly seen in the case of duplications of aspirin.
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The result is a combination leading to an identical twin drug called diaspirin, where the connection method is the no-linker mode [ 10 ] Figure 5. Fused particles are usually obtained by combining two different active compounds using the direct reactivity of functional groups of selected pharmacophores. Depending on the chemical nature of the molecules used and the type of connection formed between them, the compounds can be divided into fused stable compounds, which do not decompose in the organism and act as multifunctional innovative drugs through more than one mechanism, and the unstable ones that in physiological conditions decompose to the initial molecules [ 10 , 11 ].
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Salicylic acid and paracetamol structures can be integrated to give a non-identical twin drug called acetaminosalol Figure 6 [ 9 ]. Moreover, due to the occurrence of undesirable effects as broad gastrointestinal disorders, ibuprofen has also been chemically modified using a hydroxyl group of eugenol, thymol, and menthol, which leads to an ester association with increased lipophilicity and compounded activity and exerts protective action on the gastric mucosa [ 21 ] Figure 6.