Cytotoxic Colchicine Alkaloids: From Plants to Drugs

Plants produce and store many organic compounds like amino acids, proteins, carbohydrates, fats, and alkaloids, which are usually treated as secondary metabolites. Many alkaloids are biologically active for humans. For thousand years, extracts from plants containing alkaloids had medicinal use as drugs and they owe their powerful effects thanks to presence of alkaloids. Alkaloids have anti-inflammatory, antibacterial, analgesic, local anesthetic, hypnotic, psychotropic, antimitotic, and antitumor activity. Nowadays, alkaloids from plants are still of great interest to organic chemists, pharmacologists, biologists, biochemists, and pharmacists. Plants of Liliaceae family contain colchicine as the main alkaloid, which has cytotoxic activity. Colchicine has limited pharmacological application because of its toxicity, but many derivatives have been synthesized and their cytotoxic activity and tubulin-binding properties have been tested. Many of the synthetic derivatives showed good cytotoxic activity.


Introduction
One of the best known biologically active compounds from ancient times is colchicine (Figure 1), an alkaloid naturally occurring in Colchicum autumnale a plant of Liliaceae family and also in Gloriosa superba.In the past, extracts from these plants containing colchicine were useful in gout therapy and still are [1].The anti-gout action of colchicine could be explained by its powerful spindle toxicity [2,3].Moreover, colchicine is a useful medicine in the treatment of familial Mediterranean fever (FMF), liver cirrhosis, chronic myelocytic leukemia, Behçet disease, chondrocalcinosis and other microcrystalline arthritis also more recently in cardiovascular diseases, Sweet's syndrome, and hepatic disorders (HCC hepatocellular carcinoma) [4][5][6][7][8][9][10][11][12].
In 2009, the FDA approved colchicine for the treatment of gout and familial Mediterranean fever (FMF) [1].Recent investigations utilizing large cohorts of gout patients who have been taking colchicine for years have demonstrated novel applications within oncology, immunology, cardiology, and dermatology [4,[13][14][15][16].Some emerging dermatologic uses include the treatment of epidermolysis bullosa acquisita, leukocytoclastic vasculitis, and aphthous stomatitis.Colchicine has also anti-inflammatory and anticancer properties.Colchicine has been proven to have a fairly narrow range of effectiveness as a chemotherapy agent though it is also occasionally used in veterinary medicine to treat cancers in some animals.Nowadays, colchicine is very useful as an antimitotic agent in cancer research involving cell culture [17].Colchicine has limited medical usage because of its high toxicity [18].Because of this reason, many attempts have been made to design, synthesize new colchicine derivative and to screen them as cytotoxic agents to search more biologically active/effective compounds with lower toxicity.

Unusual chemical structure of colchinoids
Colchicine (1) is an alkaloid with unusual structure and has the whole family of structural relations.This alkaloid was isolated in 1820 by Pelletier and Caventou [26].Although listed at this point, colchicines are biogenetically very close to the isoquinoline alkaloids.Colchicines posses exocyclic N-atoms [15].Corrected structure of colchicine molecule with seven-membered C ring proposed Dewar in 1945 [27].Colchicine possesses both one stereogenic center at C7 and chirality axis, since the two rings A and C are not positioned in coplanar fashion (atropisomerism).In naturally occurring (−)-aR,7S-colchicine, the two rings (A and C) are oriented in a clockwise manner [15].

Natural, semi-synthetic, and synthetic colchicines
Many naturally occurring colchicine alkaloids (some of them are listed in Figures 3 and 4) have been converted into semi-synthetic compounds and have been prepared as potential antitumor agents.Usually starting with colchicine 1 hundreds of semi-synthetic and synthetic colchicine derivatives have been synthesized [28][29][30].
From compound 79 acetamido ─NHCOCH 3 substituent from C-7 has been removed and replaced by ═CH 2 group [56].Hybrids of vindoline, anhydrovinblastine, and vinorelbine with thiocolchicine 31 podophyllotoxin and baccatinIII have been tested in arresting cell cycle and cytotoxic activity [57].Series of thiocolchicine-podophyllotoxin conjugates have been obtained and their tubulin activity has been tested [58].

Bioactivity of colchicine and its derivatives
Colchicine 1 has been known and used from ancient times, despite its toxicity to cure acute gout attacks because of its anti-inflammatory properties.After administration of colchicine 1, it is mainly metabolized in liver via demethylation by cytochrome P450 system (isoform CYP 3A4) to 2-demethylcolchicine 2 and 3-demethylcolchicine 3 [11].Colchiceine 12 was described as a metabolite in rats produced by cytochrome P450 3A4 isoform [60], but it does not occur in humans in vivo [61].Colchicine's most common toxicity is gastrointestinal (nausea, vomiting, diarrhea, abdominal pain) which occurs during first 24 hours after overdose.Toxic effect of colchicine appears after oral administration of 7-60 mg of colchicine and is fatal, symptoms occur in about 4 h and death in about 4 days.Severe colchicine overdose may be treated with a colchicine-specific antigen-binding immunoglobulin [11].
Beside colchicine 1 has many naturally occurring derivatives many attempts have been made to discover more effective and less toxic analogs by modifying the substituents of its basic structure.
Colchicine blocks mitosis metaphase due to different anti-mitotic effects: disruption of mitotic spindle formation and second disruption of the sol-gel formation.Colchicine can also interact with lipid membranes.The interaction between colchicine and membrane results with significant alternations of both the properties of the lipid membrane and alkaloid [39].Tubulin is an α and β heterodimer initially identified as the cellular colchicine-tubulin protein [10,62].Colchicine can interact with human serum albumin, which has been studied by spectroscopic method [63,64].Study of colchicine-tubulin complex showed that colchicine binds at the location where it prevents curved tubulin from adopting a straight structure, which inhibits assembly.Microtubules are cytoskeletal polymers of tubulin involved in many cellular functions [65].Their dynamic instability is controlled by many proteins and compounds such as colchicine.
Colchicine and its biologically active derivatives, especially thiocolchicine and its derivatives, have been extensively tested on cancer cell lines for in vitro cytotoxicity, in mice, evaluated for inhibition of tubulin polymerization [66], on axonal cytoskeleton of rat peroneus nerve [67].Thiocolchicine has been studied as a potent compound to treat Peyronie's disease [68].Derivatives of thiocolchicine have been tested ex vivo to human T-lymphoblastoid (CEM) cells [69].
Many of tested colchicine derivatives and thiocolchicine derivatives obtained by partial synthesis were assayed measuring mitotic arrest in L1210 murine leukemia cell cultures [70], their binding to tubulin in vitro, their antitumor activity against the P388 lymphocytic leukemia screen in mice, and their inhibition of swelling produced in rat paws by injection with uric acid.
To measure inhibition in binding different colchicine derivatives to tubulin, many tests have been used in vitro and in vivo: CD spectra [74], radiolabeled compounds, and cancer cell lines.
Colchicine showed to be too much toxic to be used as a drug candidate for cancer diseases.

Drugs with colchicine
Besides antitumor activity, colchicine has anti-inflammatory properties.Colchicine reduces the formation of uric acid crystals in the affected joint and thereby reduces the amount of acute inflammation and pain.It also decreases the levels of uric acid in the blood or the amount that is excreted in the urine.Usually, colchicine is a major component of tablets or capsules in which in a single tablet or capsule its amount is in range of 0.5 or 0.6 mg, sometimes is used as an injection (disk problems).Usually a man/woman of 60 kg takes a dose of 0.5-4.8mg/day [82,83].Since 2008, only oral use of colchicine for patients is possible because of 50 cases of serious adverse events [84].The known medicines with colchicine are: Colchicum Dispert ® , Colcrys, Mitigare, and Colchimax.Col-Benemid or Proben-C is a drug where next to colchicine probenecid is added as uricosuric agent.

Drugs with colchicine derivatives
One of the known colchicine derivatives that has been used for the treatment of Hodgkin's lymphoma and chronic granulocytic leukemia is N-deacetyl-N-methylcolchicine, brand name is Colcemid [72].Moreover, its efficacy against melanoma and prostatic cancer has been established.
Thiocolchicoside (=glucopyranosyl derivative of the semi-synthetic 3-O-demethylthiocolchicine 41), is well-known as a muscle relaxing agent and as an anti-inflammatory drug substance [85].This compound is registered in different countries under the trade names of Colcamyl, Coltramyl, Coltrax, Miorel, and Musco-Ril.Muscle spasm is one of the main factors responsible for chronic pain, and because this particular drug reduces muscle tone, it is used in therapy for the treatment of contractures and inflammatory conditions that affect the muscular system [48].

Docking studies
A new tool for searching new potent anticancer agents is docking studies.Some years ago it became possible to study new compounds of possible biological activity by new technical methods like molecular modeling and docking studies [37,[86][87][88][89][90].

Conclusion
The way to search new colchicine derivatives especially thiocolchicine derivatives seems to be worth trying because of its promising cytotoxicity.Many new derivatives have been obtained, have been tested for many different cancer cell lines, and many of them seem to be promising anticancer agents in the future.
Cytotoxic Colchicine Alkaloids: From Plants to Drugs http://dx.doi.org/10.5772/intechopen.72622 Scientists still keep designing and synthesizing more and more colchicine derivatives for searching almost ideal anticancer agent.New methods, such as molecular modeling and docking studies, seem to be useful tool in searching for new colchicine derivatives as effective cytotoxic agents.

Figure 1 .
Figure 1.Colchicine molecule (color version available on the online version).

Figure 3 .
Figure 3. Naturally occurring colchicine derivatives (color version available on the online version).

Table 2 .
[76]bition [%] of binding radiolabeled colchicine to purified tubulin[76].Cytotoxicity liver cirrhosis, disk problems, Behçet syndrome, prevention of post-pericardial syndrome, primary biliary cirrhosis, hepatic cirrhosis, dermatitis herpetiformis, Paget's disease of bone, pseudogout, and idiopathic pulmonary fibrosis.Colchicine can be used to treat familial Mediterranean fever in children 4 years of age and older.Colchicine is available as a tablet, capsule, and a gel.In tablet form, it is available in a generic 0.6 mg tablet and as Colcrys 0.6 mg tablet.It is available as a capsule in a generic form of 0.6 mg and as Mitigare 0.6 mg capsule.There is a topical gel form of Colchicum autumnale, available as ColciGel.Colchicine is commonly administered orally, and use of the topical gel is rare.Due to toxicity of colchicine from 2009, the injectable form is not available.Dosing is dependent on age of patient and kind of illness.
More recently colchicine has been proposed as a potential drug in treatment for various conditions (except gout), what can open new way of its possible future application.Nowadays, colchicine is the useful drug in illnesses: familial Mediterranean fever (FMF),