Medicinal Chemistry II Study Guide

Fundamentals of Neurotransmission and Nervous System Physiology

• Cellular Components of the Nervous System:

  • Neurons: Structural and functional units consisting of the soma (cell body), axon, dendritic arborizations, and terminal buttons. Neurons belong to the class of excitable cells alongside skeletal/cardiac muscle cells and pancreatic $\beta$-cells.
  • Neuroglia (Glial Cells): Non-excitable cells capable of division, metabolically active, outnumbering neurons by a factor of $10$. They provide structural support, protection, insulation, metabolic supply, and homeostatic maintenance.
    • Macroglia:
      • Oligodendrocytes: Responsible for myelination of axons in the Central Nervous System (CNS).
      • Ependymal Cells: Line the cerebral ventricles and central canal of the spinal cord; secrete cerebrospinal fluid (CSF).
      • Astrocytes: The largest glial cells, forming components of the blood-brain barrier (BBB). They shuttle metabolites, nutrients, cytokines, and trace elements between blood vessels and neurons, regulate synaptogenesis, facilitate neurotransmitter reuptake, and participate in tissue repair.
    • Microglia: Resident immunocompetent cells acting as phagocytes and macrophages within the CNS.

• Neuroplasticity:

  • The CNS possesses structural and functional neuroplasticity.
  • The human brain contains approximately $100\,\text{billion}$ ($10^{11}$) neurons.
  • Each neuron participates in $1000 - 10000$ synaptic connections, forming complex neural networks.
  • Synaptic activity is enhanced by cognitive exercise and diminished by chronic stress, aging, or psychotropic substance consumption.

• Electrophysiology of Excitable Cells:

  • Resting Membrane Potential: Maintained at approximately $-70\,\text{mV}$ (interior negative relative to extracellular space).
  • Origins of Resting Potential:
    • Unequal distribution of cations ($\text{Na}^+$, $\text{K}^+$) and anions ($\text{Cl}^-$, organic protein anions).
    • Presence of non-gated (passive) leak channels.
    • High baseline membrane permeability to $\text{K}^+$ relative to $\text{Na}^+$.
  • Ion Transport Mechanisms:
    • Transmembrane protein subunits function as selective ion channels that open/close in response to specific signals.
    • Ion selectivity depends on the hydrated ionic radius. Cations undergo partial dehydration via chemical interactions with amino acid residues lining the selectivity filter of the channel pore.
    • Channel Gating Classifications: Voltage-gated, chemically/ligand-gated, and non-gated (passive leak) channels.
  • Action Potential Generation:
    • Depolarization: Rapid influx of $\text{Na}^+$ (or $\text{Ca}^{2+}$) shifts membrane potential to positive values.
    • Repolarization: Outflux of $\text{K}^+$ through open potassium channels restores negative resting potential.
    • Hyperpolarization: Influx of $\text{Cl}^-$ or extra outflow of $\text{K}^+$ drives potential more negative than $-70\,\text{mV}$, increasing threshold for activation.
    • $\text{Na}^+/\text{K}^+$ ATPase Pump: Active transport pump moving $3\,\text{Na}^+$ outward and $2\,\text{K}^+$ inward per ATP hydrolyzed to fine-tune ion gradients during resting and refractory phases.
    • Refractory Periods: Voltage-gated $\text{Na}^+$ channels transition from open ($0.1\,\text{ms}$ duration) to an inactivated state, preventing immediate re-excitation and ensuring unidirectional signal propagation (All-or-None Law).

• Synaptic Transmission:

  • Excitatory Synapses: Neurotransmitter binding triggers membrane depolarization, generating excitatory postsynaptic potentials (EPSPs).
  • Inhibitory Synapses: Neurotransmitter binding triggers hyperpolarization, generating inhibitory postsynaptic potentials (IPSPs).
  • Metabotropic Receptors: G-protein coupled receptors (GPCRs) mediate slower, complex intracellular secondary messenger cascades ($cAMP$, $IP_3/DAG$).
  • Presynaptic Autoreceptors: Mediate negative feedback pathways to self-regulate neurotransmitter synthesis and release.
  • Presynaptic Heteroreceptors: Allow neurons to respond to secondary neurotransmitters or neuromodulators.

• Neurotransmitter Criteria:

  • Synthesized and stored in specific presynaptic vesicles.
  • Released upon neuronal depolarization via $\text{Ca}^{2+}$-dependent exocytosis.
  • Exogenous application mimics physiological postsynaptic action.
  • Specific antagonists block physiological action competitively or non-competitively.
  • Specific termination mechanisms exist (enzymatic degradation or transmembrane reuptake transporters).

Glutamatergic Neurotransmission and Excitatory Amino Acid Modulators

• Excitatory Amino Acids (EAAs):

  • L-Glutamic Acid (Glutamate, Glu): The principal excitatory neurotransmitter in the mammalian brain, utilized in over $50\%$ of brain synapses.
  • L-Aspartic Acid (Aspartate, Asp): Primary excitatory neurotransmitter localized predominantly in the spinal cord.

• Glutamate Metabolic Cycle:

  • Synthesized from $\alpha$-ketoglutarate via glutamate dehydrogenase or transaminases.
  • Converted to $\gamma$-aminobutyric acid (GABA) via L-glutamic acid decarboxylase (GAD) utilizing Pyridoxal-5'-phosphate (PLP, Vitamin $B_6$) as an essential cofactor.
  • Metabolized via GABA aminotransferase (GABA-T) to succinic semialdehyde (SSA), which is oxidized by SSADH to succinate, feeding into the Krebs cycle.

• Glutamate Receptor Superfamily:

  • Ionotropic Receptors (Fast Ligand-Gated Cation Channels):
    • N-Methyl-D-Aspartate Receptors (NMDAR): Tetrameric assemblies ($NR1$, $NR2A-D$, $NR3A-B$ subunits).
    • 2-Amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic Acid Receptors (AMPAR): Mediate fast synaptic transmission.
    • Kainate Receptors (KAR): Located presynaptically and postsynaptically to modulate glutamate release.

• NMDAR Structure and Biophysical Properties:

  • Requires dual binding of L-glutamate and Glycine (strychnine-insensitive co-agonist site) for activation.
  • Voltage-Dependent Blockade: At resting membrane potential ($-70\,\text{mV}$), extracellular $\text{Mg}^{2+}$ physical blocks the channel pore. Initial membrane depolarization mediated by postsynaptic AMPAR activation repels $\text{Mg}^{2+}$, permitting high $\text{Ca}^{2+}$ and $\text{Na}^+$ conductances.
  • Synaptic Plasticity: $\text{Ca}^{2+}$ influx triggers intracellular enzymatic cascades ($CaMKII$, $PKC$) that promote Long-Term Potentiation (LTP)—the biophysical basis for learning and memory formation. Synchronized low-frequency stimulation leads to Long-Term Depression (LTD).
  • Excitotoxicity: Pathological overstimulation of NMDARs leads to excessive $\text{Ca}^{2+}$ influx, mitochondrial overload, reactive oxygen species (ROS) generation, activation of apoptotic proteases (caspases, calpains), and neuronal cell death. Implicated in acute brain injury (ischemic stroke, trauma) and chronic neurodegenerative diseases (Alzheimer's, Parkinson's, Huntington's).

• Pharmacological Modulators of the NMDA Receptor:

*   **Memantine (1-amino-3,5-dimethyladamantane):**
    *   **Mechanism:** Low-to-moderate affinity, uncompetitive, voltage-dependent NMDAR open-channel blocker. Binds near the $\text{Mg}^{2+}$ binding site inside the channel pore.
    *   **Therapeutic Selective Action:** During transient high-voltage synaptic transmission (physiological signal), memantine rapidly dissociates from the channel. During tonic, low-voltage, continuous Extrasynaptic NMDAR overactivation (pathological excitotoxicity), memantine remains bound, selectively blocking neurotoxicity while preserving cognitive function.
    *   **Indication:** Treatment of moderate-to-severe Alzheimer's disease.
    *   **Synthesis (Ritter Reaction):** Reaction of 1,3-dimethyladamantane with bromine, followed by reaction with acetonitrile in sulfuric acid to yield the acetamide intermediate. Hydrolysis with potassium hydroxide in diethylene glycol at $200\,^\circ\text{C}$ yields memantine, isolated as its hydrochloride salt.
*   **Amantadine:** Aminoadamantane derivative acting as a weak NMDAR antagonist and dopaminergic enhancer; used in Parkinson's disease dyskinesias.
*   **Ketamine & Esketamine:**
    *   **Chemical Structure:** $(R,S)$-2-(2-chlorophenyl)-2-(methylamino)cyclohexan-1-one.
    *   **Mechanism:** Non-competitive NMDAR channel blocker.
    *   **Rapid-Acting Antidepressant Effect:** Low-dose ketamine blocks NMDARs on GABAergic inhibitory interneurons, leading to disinhibition of presynaptic glutamatergic neurons. The resulting burst of glutamate release activates postsynaptic AMPARs, triggering $mTORC1$ and $BDNF$ (Brain-Derived Neurotrophic Factor) signaling cascades, promoting rapid synaptogenesis within $24\,\text{hours}$ lasting up to $1\,\text{week}$.
    *   **Indication:** $S(+)$-Esketamine nasal spray is approved for Treatment-Resistant Depression (TRD) and Major Depressive Disorder (MDD) with acute suicidal ideation.
    *   **Synthesis:** Reaction of 2-chlorobenzonitrile with $o$-chlorophenylmagnesium bromide to form (2-chlorophenyl)(1-hydroxycyclopentyl)methanone, followed by bromination, reaction with methylamine, and thermal rearrangement in aqueous $HCl$.
*   **Phencyclidine (PCP):** High-affinity non-competitive NMDAR blocker; psychotomimetic causing severe delirium, hallucinations, and addiction.

• AMPA Receptor Modulators:
  • Positive Allosteric Modulators (AMPAkines): Pyrrolidone derivatives such as Piracetam and Aniracetam (1-(4-methoxybenzoyl)-2-pyrrolidinone). Enhance glutamatergic and cholinergic transmission, functioning as nootropics and neuroprotectants.
  • Synthesis of Aniracetam: Reaction of 2-pyrrolidone with 4-methoxybenzoyl chloride in the presence of base ($NaOH$ or $SOCl_2$).
  • Negative Allosteric Modulators: Perampanel: 2-(2-oxo-1-phenyl-5-pyridin-2-yl-1,2-dihydropyridin-3-yl)benzonitrile. Non-competitive AMPAR antagonist that prevents $Na^+$ channel opening, reducing postsynaptic hyper-excitability; indicated as an antiepileptic.

GABAergic Neurotransmission and Inhibitory Pathway Modulation

• Gamma-Aminobutyric Acid (GABA):

  • The primary inhibitory neurotransmitter in the CNS, present in approximately $30 - 40\%$ of brain synapses.
  • Regulates motor control, anxiety, circadian rhythm, seizure threshold, and muscle tone.
  • Does not cross the blood-brain barrier in systemic circulation.

• GABA Metabolism & Enzymatic Targets:

  • Biosynthesis: Synthesized exclusively in GABAergic neurons from L-glutamate by L-Glutamate Decarboxylase (GAD), which requires Pyridoxal-5'-phosphate (PLP, Vitamin $B_6$) as a cofactor.

*   **Enzymatic Degradation:**
    *   **GABA Aminotransferase (GABA-T):** A PLP-dependent enzyme that transaminates GABA into Succinic Semialdehyde (SSA) while converting $\alpha$-ketoglutarate to L-glutamate.
    *   **Catalytic Mechanism:** Lys329 forms a Schiff base with PLP. GABA replaces Lys329 to form a GABA-PLP imine. Lys329 abstracts the $\gamma$-proton, generating a PMP-imine intermediate. Enzymatic hydrolysis yields SSA and Pyridoxamine Phosphate (PMP).
    *   **Succinic Semialdehyde Dehydrogenase (SSADH):** Oxidizes SSA into succinic acid using $NADP^+$, which enters the Krebs cycle.

• Pharmacological Modulators of GABA Synthesis and Metabolism:
  • Sodium Valproate (Sodium 2-propylpentanoate):
    • Inhibits GABA-T and SSADH, increasing endogenous CNS GABA concentration. Blocks voltage-gated $Na^+$ and $T$-type $Ca^{2+}$ channels.
    • Adverse Effects: Sedation, weight gain, hepatotoxicity. Strictly contraindicated in pregnancy due to severe teratogenicity (neural tube defects).
  • Gabaculine: 5-amino-1,3-cyclohexadienylcarboxylic acid, isolated from Streptomyces toyocaensis. Potent, irreversible GABA-T inhibitor that reacts with PLP to form a stable 3-aminobenzoic acid adduct. Used as a biochemical tool.

*   **Vigabatrin ($\gamma$-VinylGABA):**
    *   **Structure & Design:** $(R,S)$-4-aminohex-5-enoic acid. Rational design replacing the $\gamma$-position with a vinyl group. Lipophilicity is increased and amino $pKa$ reduced relative to GABA, facilitating uptake via the L-amino acid transporter (LAT1).
    *   **Mechanism:** Mechanism-based suicide inhibitor of GABA-T. PLP forms a Schiff base with vigabatrin; catalytic proton abstraction triggers nucleophilic attack of an active site residue onto the vinyl group, irreversibly inactivating GABA-T.
    *   **Indications:** Treatment-resistant focal seizures and infantile spasms.
    *   **Black Box Warning / Caveat:** Causes irreversible concentric visual field constriction (retinal toxicity) in $30 - 40\%$ of patients due to $GABA_C$ receptor overactivation from GABA accumulation in the retina; requires baseline and periodic ophthalmological screening.
*   **Gabapentinoids (Gabapentin & Pregabalin):**
    *   **Gabapentin:** 2-[1-(aminomethyl)cyclohexyl]acetic acid.
    *   **Pregabalin:** $(S)$-3-(aminomethyl)-5-methylhexanoic acid ($(S)$-3-isobutyl-GABA).
    *   **Mechanism:** Do NOT bind directly to $GABA_A$ or $GABA_B$ receptors, nor do they inhibit GABA-T or GAD. They selectively bind to the $\alpha_2\delta-1$ auxiliary subunit of presynaptic High-Voltage Activated (HVA) voltage-gated calcium channels (VGCCs) in the CNS, decreasing $Ca^{2+}$ influx and inhibiting downstream vesicular release of excitatory neurotransmitters (glutamate, substance P, CGRP).
    *   **Indications:** Focal epilepsy, neuropathic pain (diabetic neuropathy, post-herpetic neuralgia), fibromyalgia, and Generalized Anxiety Disorder (GAD).
    *   **Gabapentin Enacarbil:** An $N$-isobutyryloxyethoxycarbamate ester prodrug of gabapentin that utilizes intestinal nutrient transporters (MCT1 and SMVT) to double oral bioavailability and eliminate transporter saturation kinetics.
*   **GABA Reuptake Inhibitors (GAT-1 Inhibitors):**
    *   **Nipecotic Acid:** 3-piperidinecarboxylic acid. Potent inhibitor of neuronal and glial GAT-1 transporters; highly zwitterionic and unable to cross the blood-brain barrier.
    *   **Tiagabine:** $(3R)$-1-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-piperidinecarboxylic acid. Formed by conjugating the nipecotic acid polar head group to a lipophilic diarylbutenyl anchor via an ester/ether linker. Selectively inhibits GAT-1, elevating synaptic GABA concentrations. Used as adjunctive therapy for focal seizures.

• GABA Receptor Superfamily Classification:

*   **$GABA_A$ Receptor:**
    *   Ionotropic ligand-gated $Cl^-$ channel belonging to the Cys-loop receptor superfamily. Pentameric assembly composed of 5 subunits chosen from $\alpha_{1-6}$, $\beta_{1-3}$, $\gamma_{1-3}$, $\delta$, $\epsilon$, $\theta$, $\pi$.
    *   **Prototypical Composition:** $2\alpha : 2\beta : 1\gamma$. Orthosteric GABA binding sites are located at the two $\alpha/\beta$ subunit interfaces.
    *   **Gating Mechanism:** In the unliganded state, 5 bulky Leucine residues (one per subunit) form an hydrophobic gate blocking the pore. Dual GABA binding induces conformational rotation, moving Leucine residues away and lining the channel with polar Serine and Threonine residues, permitting rapid $Cl^-$ influx, postsynaptic membrane hyperpolarization, and inhibition.
    *   **Subunit Topography & Selectivity:**
        *   **Synaptic Receptors ($\,\alpha_1\beta_2\gamma_2, \alpha_2\beta_3\gamma_2, \alpha_3\beta\gamma_2$):** Mediate phasic inhibition; sensitive to benzodiazepines.
        *   **Extrasynaptic Receptors ($\,\alpha_4\beta\delta, \alpha_6\beta\delta, \alpha_5\beta\gamma_2$):** Mediate tonic baseline inhibition; insensitive to classical benzodiazepines.
    *   **Orthosteric $GABA_A$ Agonists:** Muscimol (isolated from *Amanita muscaria*, hallucinogenic) and THIP (Gavoxadol, $4,5,6,7$-tetrahydroisoxazolo[5,4-$c$]pyridin-3-ol, sleep-promoting agent withdrawn due to side effects).
*   **$GABA_B$ Receptor:**
    *   Metabotropic $G_i/G_o$-protein coupled receptor existing as a $GABA_{B1}/GABA_{B2}$ heterodimer. GABA binds $GABA_{B1}$; $GABA_{B2}$ couples to G-proteins, inhibiting presynaptic $Ca^{2+}$ channels and opening postsynaptic inward-rectifying $K^+$ channels ($GIRK$), producing slow, prolonged IPSPs.
    *   **Baclofen:** $(R,S)$-4-amino-3-(4-chlorophenyl)butanoic acid. Selective orthosteric $GABA_B$ agonist used orally or intrathecally (baclofen pump) for spasticity in multiple sclerosis and spinal injury.
*   **$GABA_C$ ($GABA_{A-rho}$) Receptor:** Homomeric assemblies of $\rho_{1-3}$ subunits localized mainly in the retina; insensitive to bicuculline and benzodiazepines.

Hypnotics, Sedatives, and Anxiolytics

• Pathology & Physiological Overview:

  • Anxiety Disorders: Pathological state of fear and hyper-arousal involving dysregulation of the amygdala, hippocampus, and hypothalamus. Includes Panic Disorder, Generalized Anxiety Disorder (GAD), Social Anxiety Disorder, and PTSD.
  • Insomnia: Chronic disruption of sleep initiation, duration, or quality. Controlled by the reticular activating system and suprachiasmatic nucleus (SCN).

• Benzodiazepines (BZDs):

  • Mechanism of Action: Positive Allosteric Modulators (PAMs) of $GABA_A$ receptors. Bind to the extracellular $\alpha/\gamma_2$ subunit interface ($\,\alpha_1, \alpha_2, \alpha_3, \alpha_5$-containing receptors). BZD binding increases the frequency of $Cl^-$ channel opening in response to GABA (without altering channel conductance or open duration).
  • Discovery History: Synthetic discovery of Chlordiazepoxide (Librium) by Leo Sternbach at Hoffmann-La Roche (1954–1959). Reaction of 2-amino-5-chlorobenzophenone oxime with chloroacetyl chloride produced a quinazoline $N$-oxide intermediate; subsequent reaction with methylamine yielded unexpected ring expansion into the 1,4-benzodiazepine core.

• Representative Benzodiazepine Agents:

  • Diazepam (Valium, Stedon): 7-chloro-1-methyl-5-phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2-one. Highly lipophilic, fast onset, long duration of action. Metabolized by hepatic CYP3A4/CYP2C19 to active metabolites: $N$-desmethyldiazepam (nordiazepam, $t_{1/2} = 36-200\,\text{h}$), temazepam, and oxazepam.
  • Lorazepam (Tavor): 7-chloro-5-(2-chlorophenyl)-3-hydroxy-1,3-dihydro-2H-1,4-benzodiazepin-2-one. Intermediate-acting; direct phase II glucuronidation at $C_3$-OH yields inactive metabolites, making it safer in hepatic impairment and elderly patients.
  • Flunitrazepam (Hipnostedon): 7-nitro-5-(2-fluorophenyl)-1-methyl derivative; potent hypnotic with high risk of abuse and anterograde amnesia.
  • Quazepam: 2-thione-substituted BZD exhibiting functional selectivity for $\alpha_1$-containing $GABA_A$ receptors (Type I BZD sites), acting as a hypnotic with reduced ataxia.
  • Clobazam: 1,5-benzodiazepine; binds selectively to Type II $GABA_A$ receptors ($\,\alpha_2$-subunit), demonstrating high anticonvulsant and anxiolytic efficacy with minimal sedation.

*   **Midazolam:** 8-chloro-6-(2-fluorophenyl)-1-methyl-4H-imidazo[1,5-a][1,4]benzodiazepine. Water-soluble formulation due to pH-dependent reversible ring opening of the 1,4-diazepine ring at $pH < 4$. At physiological $pH\,7.4$, intramolecular cyclization restores the lipophilic closed ring structure, allowing rapid BBB penetration ($t_{1/2} = 10\,\text{min}$). Used in status epilepticus and pre-anesthetic sedation.
*   **Triazolam & Alprazolam (Xanax):** 1,2,4-triazolo-fused benzodiazepines. Ultra-short to short-acting due to rapid $\alpha$-hydroxylation of the $1-\text{CH}_3$ group by CYP3A4. Concomitant consumption of grapefruit juice (CYP3A4 inhibitor) increases blood levels by $30\%$.
*   **Flumazenil:** Ethyl 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate. BZD antagonist (null allosteric modulator); competitively displaces BZD PAMs and NAMs without altering baseline $Cl^-$ flux. Used as an antidote for BZD overdose and post-anesthesia reversal.

• Structure-Activity Relationships (SAR) of Benzodiazepines:

  • Ring A (Aromatic Ring): Aromatic ring essential ($\,\pi-\pi\,$ stacking). An electronegative substituent at position 7 ($-\text{Cl}, -\text{NO}_2, -\text{CF}_3$) is required for activity; substitution at positions 8 or 9 drastically reduces potency.
  • Ring B (7-Membered Diazepine Ring):
    • $N_1$ position: Small alkyl ($-\text{CH}_3$, cyclopropylmethyl) allowed. Fused electron-rich heteroaromatic rings (triazole, imidazole) at the $a$-face increase receptor affinity.
    • $C_2$ position: Carbonyl oxygen essential for hydrogen bonding.
    • $C_3$ position: Proton required. Hydroxylation ($-\text{OH}$) preserves activity while accelerating Phase II clearance (shortens half-life).
  • Ring C (5-Phenyl Ring): Phenyl ring enhances binding. $2'$-halogen ($-\text{F}, -\text{Cl}$) substitution increases activity; $4'$-substitution decreases activity.

• **Non-Benzodiazepine $GABA_A$ Agonists (