Alkaloids in Medicine

Alkaloids, one of the largest groups of natural products, represent a highly diverse group of chemical entities. Most alkaloids with complex nitrogen heterocyclic structures and a wide range of physiological activities have specific defensive and therapeutic properties. In plants, alkaloids protect plants from predators and regulate their growth. Therapeutically, alkaloids are particularly well known as antitumor agents, anesthetics, antibacterial agents, and anti-inflammatory agents. Alkaloids provided unique lead compounds for medicine. They are water soluble under acidic conditions and lipid soluble under neural and basic conditions. This is especially important for dissolution in protonated form and membrane permeation in deprotonated form. At present, there are hundreds of alkaloid drugs used in clinical practice. With the continuous development of science and technology, more and more alkaloids are expected to be used in the treatment of various diseases.

The role of alkaloids in medicine

Alkaloids play an essential role in human medicine. Its role includes but is not limited to the following.

• Antitumor effect

The search for highly effective and less toxic antitumor drugs from natural drugs has become a topic of interest for scholars at home and abroad, among which natural alkaloids are a promising class of active substances. For example, alkaloids vincristine and vinblastine are used as chemotherapeutic agent in the treatment of many cancer types. Colchicine inhibits the proliferation of cancer cells.

Figure 1. The structure of vincristine (A) and vinblastine (B).

• Analgesic effect

Most alkaloids have analgesic effects by acting on the nervous system and attenuating or blocking the transmission of nociceptive signals. Examples include the 4-quinolone alkaloids in pepper and strychnine alkaloids.

Figure 2. The structure of 4-quinolone (A) and strychnine (B).

• Antibacterial effect

Alkaloids are an important source of new and highly effective antibacterial agents, which can be used in the fight against the growing drug resistance due to the emergence of multidrug- and extensively drug-resistant bacterial phenotypes. For example, matrine has significant inhibitory effects on Aspergillus, Escherichia coli and Pseudomonas aeruginosa in dysentery.

Figure 3. The structure of matrine.

• Anti-inflammatory effect

Many studies have shown the anti-inflammatory potential of alkaloids found in plants that act on major inflammatory control pathways. For example, indole alkaloids such as brucine and brucine-N-oxide have significant anti-inflammatory properties. Among the isoquinolines, berberine is one of the major components of Coptis chinesis, which is frequently utilized in Chinese herbal drugs to treat inflammatory reactions.

Figure 4. The structure of brucine (A) and berberine (B).

• Other effects

Studies have shown that alkaloids also have other aspects of action. For example, quinine is a powerful antimalarial agent derived from the Cinchona species. Another alkaloid from Cinchona species is quinidine which has medical application as treatment of irregular rhythms of the heartbeat or arrhythmias. Ergonovine, an alkaloid from the fungus Claviceps purpurea, and the second alkaloid ephedrine isolated from Ephedra species both act as blood vessel constrictors. Also, ephedrine is used in bronchial asthma and to relieve discomfort of hay fever, sinusitis, and common colds.

Figure 5. The structure of quinine (A), quinidine (B), ergonovine (C) and ephedrine (D).

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