Pharmacognosy II: Comprehensive Notes on Isoquinoline Alkaloids

Classification and Characteristics of Isoquinoline Alkaloids

Definition and General Structure: * Isoquinoline alkaloids are characterized by the presence of an isoquinoline nucleus, which is a benzene ring fused to a pyridine ring. * They are structural isomers of quinoline alkaloids. * These compounds are predominantly found in specific plant families, specifically: * Papaveraceae (the poppy family). * Berberidaceae (the barberry family). * Ranunculaceae. * Species within these families have a long history of use in traditional medicine.
Summary of Representative Isoquinoline Alkaloid Subclasses: * Benzylisoquinolines: Papaverine (found in Opium). * Morphinans: Morphine, codeine, and thebaine (found in Opium). * Protopines: Protopine (found in Fumariae herba). * Benzophenanthridines: Sanguinarine (found in Chelidonii herba). * Phthalyltetrahydroisoquinolines: Noscapine (also known as narcotine, found in Opium) and Hydrastine (found in Hydrastidis rhizoma). * Secophthalylisoquinolines: Narceine (found in Opium). * Protoberberines: Berberine (found in Hydrastidis rhizoma), Palmatine, columbamine, and jatrorhizine (found in Colombo Radix). * Aporfines: Boldine (found in Boldi folium). * Bisbenzyltetrahydroisoquinolines: Tubocurarine (found in Curare). * Monoterpenoid isoquinolines: Emetine, cephaeline, and psychotrine (found in Ipecacuanhae radix).

Benzylisoquinoline Alkaloids: Papaverine

Structural Characteristics: * Benzylisoquinolines represent the fundamental subclass of this group. * They contain a $1$ -benzyl- $1,2,3,4$ -tetrahydroisoquinoline scaffold, or its fully aromatic isoquinoline counterpart. * These compounds typically feature multiple oxygenation substituents, such as hydroxy ( $- ext{OH}$ ) or methoxy ( $- ext{OCH}_3$ ) groups, on the aromatic rings. * Papaverine specifically possesses a fully aromatic isoquinoline core with methoxy substitutions.
Pharmacological Activities of Papaverine: * Papaverine functions as a non-selective phosphodiesterase inhibitor. * This inhibition leads to increased levels of cyclic adenosine monophosphate ( $ext{cAMP}$ ) and cyclic guanosine monophosphate ( $ext{cGMP}$ ), which results in the relaxation of smooth muscles. * It is used clinically as a vasodilator and antispasmodic to relax blood vessels and gastrointestinal smooth muscles, thereby alleviating spasms. * Clinical Applications: Treatment of visceral spasm, vasospasm, and intracavernosal injection for erectile dysfunction. * Important Distinction: Despite originating from the same plant as morphine, papaverine has no analgesic (pain-killing) properties and is pharmacologically distinct from morphine-type opiates.

Morphinan Alkaloids: Morphine, Codeine, and Thebaine

Chemical Structure: * These alkaloids are derived from $(S)$ -reticuline through intramolecular cyclization. * This process forms a phenanthrene system (three fused benzene rings). * Morphine features a pentacyclic structure with a tertiary nitrogen bridge, known as the morphinan skeleton.
Pharmacology and Clinical Use: * Morphinan alkaloids are potent analgesics that act on opioid receptors. * Morphine: A strong opioid agonist used for severe pain (e.g., post-surgical or cancer pain). It produces pain relief, sedation, and euphoria, but can cause respiratory depression and dependence. * Codeine: A milder opioid and a prodrug that is partially metabolized to morphine. It is used as an analgesic for moderate pain and as a cough suppressant. * Thebaine: It is not used directly in medicine because it is a convulsant; however, it serves as an essential precursor for semisynthetic opioids like oxycodone and buprenorphine. * These substances are heavily controlled due to their abuse potential.

Opium: Source and Pharmaceutical Standards

Botanical Origin: * Plant name: Papaver somniferum L. * Drug: Opium crudum (as defined by PhEur). * Family: Papaveraceae.
Opium Collection Process: * Raw opium is the air-dried latex obtained by making incisions in the unripe capsules of the poppy plant. * Incisions are made early in the morning on immature capsules. * The depth of the incision must reach the mesocarp layer to stimulate latex flow without damaging the inner capsule wall. * The exuded whitish fluid darkens and turns brown by evening due to oxidation and drying.
European Pharmacopoeia (PhEur) Standards for Opium Forms: * Raw Opium (Opium crudum): Must contain $\ge 10\, \%$ morphine and $\ge 2\, \%$ codeine. * Standardized Dry Extract (Opii extractum siccum normatum): Must contain $19.6-20.4\, \%$ morphine and $\ge 2\, \%$ codeine. * Standardized Powdered Opium (Opii pulvis normatus): Prepared below $70\, ^{\circ}\text{C}$ . Must contain $9.5-10.5\, \%$ morphine and $\ge 1\, \%$ codeine. * Standardized Tincture of Opium (Opii tinctura normata): An ethanolic ( $70\, \% \, v/v$ ) extract. Must contain $0.95-1.05\, \%$ morphine and $\ge 0.1\, \%$ codeine.
Opium Taxonomy (Subspecies): * Papaver somniferum subsp. spontaneum: Capsule pores open at maturity for natural seed dispersal. * Papaver somniferum subsp. anatolicum: Capsules remain closed at maturity. * Sub-varieties: * var. album: White flowers. * var. nigrum: Purple flowers. * var. setigerum: Dark purple flowers. * var. glabrum: Red-purple flowers.
Chemical Composition of Opium: * Total alkaloid content: approximately $20-30\, \%$ . * Morphinan Alkaloids: Morphine ( $10-12\, \%$ ), Codeine ( $2.5-5\, \%$ ), and Thebaine ( $\approx 1\, \%$ ). * Phthalideisoquinoline Alkaloids: Noscapine ( $2-10\, \%$ ). * Benzylisoquinoline Alkaloids: Papaverine ( $0.5-1.5\, \%$ ). * Secophthalideisoquinoline Alkaloids: Narceine.

History and Regulation of Opium in Turkey

Historical Background: * Opium poppy cultivation in Anatolia dates back to the Hittite period. * Between $1964$ and $1971$ , the number of Turkish provinces allowed to cultivate poppy was reduced from $42$ to $4$ .
The US Ban and Socioeconomic Impact: * The US alleged that $80\, \%$ of heroin entering the US originated from Turkish opium processed in Marseille, despite weak scientific evidence. * On June $29$ , $1971$ , the Turkish government banned all poppy cultivation under American pressure. * This affected approximately $1.5$ million Turkish citizens who relied on poppy farming. * US President Nixon promised $\$35$ million in compensation ( $\$15$ million for farmers and $\$20$ million for diversification), but the US Senate later rejected these aid proposals.
Policy Reversal and Current Model: * On July $1$ , $1974$ , cultivation was restored in $7$ provinces for pharmaceutical needs. * On December $6$ , $1974$ , scoring (latex collection by incision) was prohibited. * Turkey shifted to an unscored capsule harvesting model, which reduced illicit diversion and established the "licit capsule-based opium production model."

Protoberberine Alkaloids: Berberine

Chemical Structure and Physical Properties: * Characterized by a tetracyclic cationic skeleton ("berberine-type"). * Features a fully conjugated planar system with a permanently charged quaternary nitrogen at physiological $\text{pH}$ . * Berberine is a fluorescent, yellow pigment due to its structure and methoxy substitutions on rings A and D. * Its rigid planar structure allows it to intercalate with biomolecules like DNA.
Pharmacological and Metabolic Effects: * Metabolic Regulation: Exhibits hypoglycemic (blood glucose-lowering) and hypolipidemic (lipid-lowering) actions. * Mechanism: Activates AMP-activated protein kinase ( $\text{AMPK}$ ), improving insulin sensitivity and modulating gut microbiota. * Diabetes: Shown to lower $\text{HbA1c}$ in type 2 diabetic patients (efficacy comparable to metformin). * Anti-aging: Enhances mitochondrial function by increasing $\text{ATP}$ content and upregulating the transcriptional activity of $\text{PGC-1}\alpha$ (a regulator of mitochondrial biogenesis). * Anti-inflammatory: Inhibits \text{NF-\kappa B} and $\text{COX-2}$ . * Anticancer: Can induce cell cycle arrest and apoptosis.
Clinical Considerations for Berberis vulgaris L. (Radix Berberidis): * Usage: Used in Persian, European, and Ayurvedic medicine for fever, diarrhea, and infections. * Dosage: Generally $200-500\, \text{mg}$ of isolated berberine, $2-3$ times daily. * Contraindications: Pregnancy and lactation (uterine stimulant); avoid in neonates (risk of kernicterus due to bilirubin displacement). * Interactions: May interact with cytochrome $\text{P450}$ enzymes and $\text{P}$ -glycoprotein substrates.

Bisbenzylisoquinoline Alkaloids: Tubocurarine and Curare

Structure: * Dimeric isoquinolines consisting of two benzylisoquinoline units linked by ether or carbon bonds. * Tubocurarine: Consists of two tetrahydroisoquinoline moieties connected by a large alkyl bridge, with quaternary ammonium centers at each end.
Pharmacology of d-Tubocurarine: * A competitive antagonist at nicotinic acetylcholine receptors ( $\text{nAChRs}$ ) at the neuromuscular junction. * Causes skeletal muscle paralysis (non-depolarizing muscle blocker). * History: Used as a muscle relaxant in surgical anesthesia, tetanus treatment, and electroshock preparation. * Toxicity: High doses paralyze the diaphragm, causing respiratory arrest.
Ethnobotany of Curare: * Source plants: Chondrodendron sp. (Menispermaceae) and Strychnos sp. (Loganiaceae). * Used as a traditional arrow poison by South American tribes for hunting. * Ineffective when ingested orally but highly toxic when administered parenterally. * Antidote: Physostigmine.

Benzophenanthridine Alkaloids: Chelidonium majus

Chemical Structure: * Features a polycyclic aromatic phenanthridine system fused to an isoquinoline. * Usually exists as quaternary salts.
Chelidonium majus L. (Greater Celandine): * Drog: Chelidonii herba (Papaveraceae family). * Alkaloids: Chelidonine (marker compound), Sanguinarine, Coptisine, and Protopine. * Requirement: Total alkaloid content (as chelidonine) must be $\ge 0.6\, \%$ in the dried herb.
Clinical and Folk Uses: * Dermatology: Topical application of orange latex (sap) for warts due to antiviral and keratolytic effects. * Gastrointestinal: Treatment of spasms in the upper GI tract and gallbladder; shows spasmolytic, choleretic, and cholagogue activity. * Antimicrobial: Sanguinarine was used in dental products, though discontinued due to oral leukoplakia risks. * Chelerythrine: A potent protein kinase C inhibitor used in research. * Safety Note: Not recommended as an infusion due to hepatotoxicity risk and dosing difficulty. High doses can cause GI cramps, dizziness, and hematuria.

Monoterpenoid Isoquinolines: Ipecac Alkaloids

Source and Composition: * Plant names: Cephaelis ipecacuanha and Cephaelis acuminata. * Drug: Ipecacuanhae Radix (Rubiaceae family). * Major Alkaloids: Emetine (major component) and Cephaeline. * Total Alkaloid Content: $2-3.5\, \%$ .
Pharmacological Profile: * Cephaeline: Responsible for potent emetic (vomiting) effects by irritating the gastric mucosa and stimulating the central chemoreceptor trigger zone (CTZ). * Emetine: Historically used as an amoebicidal agent (to treat Entamoeba histolytica) and an expectorant. It inhibits protein synthesis in parasites.
Toxicity and Current Status: * Emetine is highly toxic, causing cardiotoxicity (arrhythmia, tachycardia, hypotension) and skeletal muscle damage. * Historically used as "Syrup of Ipecac" to induce vomiting after poisoning. * Modern Status: Now obsolete in clinical practice and not recommended for routine treatment of poisoning due to systemic toxicity.