Convallatoxin is a glycoside extracted from '' Convallaria majalis''.

History

Convallatoxin is a natural cardiac glycoside that can be found, among others, in the plant lily of the valley (''Convallaria majalis''). Legend says that Apollo gave this plant to Asclepios, the Greek god of healing. Lily of the valley has indeed been used medicinally to treat illness, all going back to medieval times. Convallatoxin has a similar therapeutic target and effect as digitalis, so it was used by medieval herbalists as a substitute for foxglove in treatment. It is mostly administered because it strengthens the heartbeat, while also slowing and regulating the heart rate. In 2011, the lily of the valley was used in the US television show

Breaking Bad ''Breaking Bad'' is an American crime drama television series created and produced by Vince Gilligan. Set and filmed in Albuquerque, New Mexico, the series follows Walter White (Bryan Cranston), an underpaid, overqualified, and dispirited hig ...

. This made the plant, and its compound convallatoxin, quite well known by the general public as fatal.

Structure and reactivity

The systematic name of the organic compound convallatoxin is as follows: (1''R'',3a''S'',3b''R'',5a''S'',7''S'',9a''S'',9b''S'',11a''R'')-3a,5a-dihydroxy-11a-methyl-1-(5-oxo-2,5-dihydrofuran-3-yl)-7-hexadecahydro-9a''H''-cyclopenta 'a''henanthrene-9a-carbaldehyde. A shorter semisystematic IUPAC name is 5,14-dihydroxy-19-oxo-3β-(α-L-rhamnopyranosyloxy)-5β-card-20(22)-enolide. The structure of convallatoxin consists of strophanthidin and has a 6-deoxy-α-L-mannopyranosyl group attached at position 3. Convallatoxin can donate a

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

at five places and it can accept a hydrogen bond on ten accounts. Its melting point lies between 235 and 242 degrees Celsius and the compound is soluble in alcohol, acetone and slightly in chloroform, ethyl acetate and water. Since convallatoxin is structurally similar to

digoxin Digoxin (better known as Digitalis), sold under the brand name Lanoxin among others, is a medication used to treat various heart conditions. Most frequently it is used for atrial fibrillation, atrial flutter, and heart failure. Digoxin is on ...

, research has been done to determine if convallatoxin in serum can be detected with LOCI digoxin assays. This showed that the compound has significant cross-reactivity with the used antibody and that it causes bidirectional interference in the digoxin assay. So, convallatoxin can indeed be detected with a LOCI digoxin assay. It may also be possible that convallatoxin cross-reacts with the antidigoxin antibody used in other commercially available digoxin assays, but this should be investigated further. Furthermore, the antigen Digibind does also bind convallatoxin in vitro. This could possibly be used in treatment of convallatoxin poisoning.

Synthesis

Even though convallatoxin can be found in nature, it is also synthesized by manufacturers. This can be done via the Koenigs-Knorr method, in which strophanthidin is glycosylated with 2,3,4-tri-''O''-acetyl-α-L-rhamnopyranosyl bromide. These two compounds are the precursors of convallatoxin. After alkaline hydrolysis, extraction from strophanthidin residues and crystallization of isopropanol, the reaction product is liberated. This reaction product is convallatoxin. When using 10 grams of strophanthidin, 13.6 grams of convallatoxin can be produced.

Mechanism of action and efficacy

Convallatoxin is a digitalis like compound (DLC), which is mainly used as a cardiac glycoside since it can inhibit the Na⁺,K⁺-ATPase in congestive

heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...

or arrythmias, which causes an inotropic effect, same as many other digitalis like compounds. The Na⁺,K⁺-ATPase creates the ion gradient between the intra- and extracellular domains of a cell. It does this by transporting three sodium ions out of and two potassium ions into the cell. If the Na⁺,K⁺-ATPase is inhibited, potassium will accumulate in the cell, leading to hyperkalaemia and neuromuscular dysfunction in the heart. Potassium accumulation will inhibit the calcium from exiting the cell, causing calcium accumulation as well. If calcium accumulates in cardiac myocytes, the uptake of calcium into the sarcoplasmic reticulum (SR) is increased. Thus, when stimulation of the cardiac muscle occurs, the SR releases higher levels of calcium, which increases the contractility of the myocytes. The increased release of calcium also increases the refractory period of the atrioventricular (AV) node, regulating the heart beat cycle in patients with arrythmia. In

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

, colon and breast cancer cells, convallatoxin shows great effects at nano doses. It has been shown to inhibit cell proliferation, invasion and migration of cancer cells. The underlying mechanisms of this are not fully known. However, it has been demonstrated that convallatoxin induces

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

and

autophagy Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...

at a dose of 10 nM per 3 days. It was also shown to inhibit

angiogenesis Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splitting ...

through autophagy and apoptosis at concentrations of 2-4 nM. Autophagy is induced in human cervical carcinoma cells, or HeLa cells by convallatoxin blocking the mTOR signalling pathway. This signalling pathway usually inhibits autophagy in cells. Convallatoxin induces apoptosis by increasing caspase-3 and PARP cleavage. These proteins induce programmed cell death when activated by cleavage. It is not entirely clear if the induction of apoptosis and autophagy is related to the inhibitory effects of convallatoxin on the Na⁺, K⁺-ATPase pump. However, a dose of 10 nM convallatoxin can reduce A549 non small cell lung cancer cells by inhibiting the Na⁺,K⁺-ATPase. Numbers differ per experiment. In colon cancer a LD50 of 50 nM is shown. In MCF-7 derived breast cancer cells an IC50 dose of 10 nM over a long time (exposure 24 h) show 27.65 ± 8.5 or over an even longer time (exposure 72 h), 5.32 ± 0.15 are observed. There are many more potential therapeutic uses for convallatoxin, for example against

cystic fibrosis Cystic fibrosis (CF) is a rare genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. O ...

and

neurodegenerative diseases A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...

. It has also been demonstrated to inhibit viral infection and replication. For example, convallatoxin can be used as a treatment for the Human Cytomegalovirus. It will inhibit the Na⁺-K⁺-ATPase pump which decreases the sodium concentration outside the cell, and thus limiting cotransport of

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...

and sodium into the cell, disabling protein synthesis. A dose of 0.01 μM already has a great efficacy against the cytomegalovirus, but at a dose of 50 nM or less a great potency is also shown that can last up to 4 hours. Convallatoxin is thus quite an efficient drug, showing effects with small doses in treatment of multiple diseases. It is excreted by

P-glycoprotein P-glycoprotein 1 (permeability glycoprotein, abbreviated as P-gp or Pgp) also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-family B member 1 (ABCB1) or cluster of differentiation 243 (CD243) is an important protein ...

and an affinity of 1.07 ± 0.24 mM and a V_max of 5.2 ± 0.4 mmol mg/protein/min were determined. Excretion of convallatoxin is mainly by the kidneys (a clogP of about -0.7).

Metabolism

Convallatoxin is mainly

metabolised Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

in the liver by the conversion of convallatoxin into convallatoxol. For this, the aldehyde (-CHO) group attached to C¬10 is reduced to an alcohol group (-CH2OH) by

cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...

reductase (CYP450). This is a

phase I metabolism Drug metabolism is the metabolic breakdown of drugs by living organisms, usually through specialized enzymatic systems. More generally, xenobiotic metabolism (from the Greek xenos "stranger" and biotic "related to living beings") is the set o ...

reaction. However, further modification through a phase II reaction of convallatoxin has not been found. The reduction of convallatoxin increases its polarity, thus enabling the compound to be excreted more readily. This form of convallatoxin metabolism can be found in

rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot ...

s, however is not present in guinea pigs and only traces of convallatoxol can be found in cats.

Toxicity

Convallatoxin has a very small therapeutic index (40-50 nM) i.e. the margin is narrow between a therapeutic dose and an overdose giving rise to symptoms of poisoning. Even so, the

cytotoxicity Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa''). Cell physiology Treating cells ...

of convallatoxin is mainly time-dependent. At an increased plasma level, DLCs (including convallatoxin) toxicity symptoms include dizziness,

fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...

, nausea, loss of appetite, vision disturbance, vomiting,

hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...

, arrythmia, cardiac arrest,

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...

, abdominal pain and convulsions,

or death.

Effects on animals

On certain animals, convallatoxin has quite interesting effects. The lifespan of '' C. elegans'', a nematode, can be expanded by convallatoxin. About 20 μM of convallatoxin shows no toxicity and can expand the lifespan of the worm by 16.3% due to certain mechanisms, including improvement of pharyngeal pumping, locomotion, reduced lipofuscin accumulation and ROS. Where the convallatoxin has quite positive effects on nematodes, it is extra poisonous to cats. It causes nephrotoxicity and acute renal failure, but at what dose exactly is not known. Symptoms are

salivation Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be e ...

, vomiting, anorexia and depression. It can be treated with

dialysis Dialysis may refer to: *Dialysis (chemistry), a process of separating molecules in solution **Electrodialysis, used to transport salt ions from one solution to another through an ion-exchange membrane under the influence of an applied electric pote ...

, when diuresis is started before the acute renal failure.

References

{{reflist Cardenolides Rhamnosides Aldehydes Plant toxins