Functional Group Modification, SAR, and Mechanism-Based Enzyme Inhibition

Functional Group Modification and Structure-Activity Relationships

Principle of Separation of Activity: In medicinal chemistry, modifying functional groups can separate a desired therapeutic effect from an undesirable side effect or different pharmacological activity. - Case Study: Carbutamide and Tolbutamide - Carbutamide ( $R = NH_2$ ): This antibacterial agent was discovered to possess an antidiabetic side effect. However, it was unsuitable for clinical use as an antidiabetic because its inherent antibacterial activity could lead to widespread bacterial resistance. - Modification: The amino group ( $NH_2$ ) of carbutamide was replaced by a methyl group ( $CH_3$ ). - Tolbutamide ( $R = CH_3$ ): The resulting compound, tolbutamide, retained the antidiabetic activity but lacked the antibacterial activity. This successfully separated the two biological effects. - Chemical Structure: The general scaffold is $R-SO_2NHCNHCH_2CH_2CH_2CH_3$ .
Expert Intuition and Rational Design: - Experienced medicinal chemists can often predict which functional group will elicit a specific effect based on previous studies and chemical analogies. - Case Study: Chlorothiazide and Diazoxide - Chlorothiazide: An antihypertensive drug that also possesses a strong diuretic effect (increased urine excretion). - SAR Insight: Research on sulfanilamide previously established that the primary aminosulfonyl ( $NH_2SO_2$ ) side chain group is responsible for diuretic activity. - Diazoxide: To create a drug without the diuretic side effect, diazoxide was prepared as an antihypertensive medication that lacks the diuretic activity found in chlorothiazide.
Historical Context of Molecular Activity: - The realization that there is a definitive relationship between the molecular structure of a compound and its biological activity first emerged approximately 145 years ago.

Serendipity in Drug Development

Accidental Discoveries: Many significant drugs have been discovered by chance rather than by purely rational design.
Nitroglycerin: - Application: Used to relieve the symptoms of angina pectoris (severe heart pain). - Discovery: Workers in the explosives industry who handled nitroglycerin reported suffering from severe headaches. - Investigation: Research revealed that these headaches were caused by the chemical's ability to produce a marked dilation of blood vessels. - Therapeutic Mechanism: Angina pain occurs when blood vessels are unable to supply the heart with adequate blood. Nitroglycerin relieves this pain by dilating the cardiac blood vessels. - Chemical Structure: $CH_2(ONO_2)-CH(ONO_2)-CH_2(ONO_2)$ .
The Discovery of Librium® (Chlordiazepoxide): - Context: Leo Sternbach synthesized a series of quinazoline 3-oxides, but early tests showed no pharmacological activity. - Timeline: One specific compound was not originally submitted for testing because it was a byproduct rather than the intended quinazoline 3-oxide. The project was neglected for two years. - The Accident: During a laboratory cleanup, a worker found the neglected compound. Sternbach decided to submit it for testing before discarding it. - Result: The compound exhibited potent tranquilizing properties. - Structural Investigation: Upon review, the compound was found to be a benzodiazepine 4-oxide rather than a quinazoline 3-oxide.
Evolution of Benzodiazepines: - Valium® (Diazepam, 1963): A structural modification of Librium that is approximately $10 \times$ more potent. - Rohypnol® (Flunitrazepam, 1963): Noted as one of the so-called "date-rape" drugs. - Xanax® (Alprazolam, 1970): One of the most widely prescribed medications. - Current Status: There are currently eight benzodiazepines in clinical use as tranquilizers in the United States, with approximately 15 others used abroad.

Antibiotics and Penicillin

Definition of Antibiotics: Chemical substances produced by microorganisms that, when excreted, interfere with the growth or metabolism of other microorganisms. These compounds are effective even at low concentrations.
Historical Discovery: In 1929, Alexander Fleming discovered a mold of the Penicillium species that inhibited the growth of certain bacteria.
Classification: Antibiotics are classified based on their chemical structures or the nature of their biological activity.
The Penicillins: - Nature: A mixture of natural compounds with the general molecular formula $C_{9}H_{11}N_{2}O_{4}SR$ . - Variations: The different types of natural penicillins differ only in the structure of the $R$ group. There are at least six natural penicillins: 1. Penicillin I or F (Pent-2-enylpenicillin): $R = CH_2CH=CHCH_2CH_3$ 2. Penicillin II or G (Benzyl penicillin): $R = CH_2C_6H_5$ 3. Penicillin III or X (p-hydroxybenzyl penicillin): $R = p\text{-hydroxybenzyl}$ 4. Penicillin IV or K (n-Heptyl penicillin): $R = (CH_2)_6CH_3$ 5. Dihydro-F penicillin (n-Amyl penicillin): $R = (CH_2)_4CH_3$ 6. Penicillin V (Phenoxy methyl penicillin): $R = CH_2OC_6H_5$
Other Antibiotic Classes: These antibacterials belong to different classes with distinct mechanisms of action: - Cephalosporin - Streptomycin - Tetracycline - Chloramphenicol

Drugs as Enzyme Inhibitors

Mechanism of Penicillin: Penicillin destroys bacteria by inhibiting the specific enzyme responsible for synthesizing bacterial cell walls.
Bacterial Resistance via Penicillinase: - Bacteria develop resistance by secreting an enzyme called penicillinase (a type of $\beta$ -lactamase). - Action: Penicillinase destroys penicillin by hydrolyzing its lactam ring, converting the active drug into an inactive product called penicilloic acid before it can interfere with cell wall synthesis.
Penicillinase Inhibitors: - Chemists developed drugs that inhibit penicillinase to be administered alongside penicillin. This protects the antibiotic from destruction. - These inhibitors are examples of drugs that have no therapeutic effect on their own but protect a therapeutic drug. - Sulfone Inhibitors: A penicillinase inhibitor can be a sulfone, prepared from penicillin by oxidizing the sulfur atom using a peroxyacid ( $RCOOH$ ).
Suicide Inhibition Mechanism: - Molecular Mimicry: Because the sulfone structure resembles the original penicillin antibiotic, the penicillinase enzyme accepts it as a substrate. - Pathway: The electron-withdrawing sulfone group provides an alternative pathway to simple hydrolysis, leading to the formation of a stable imine. - Covalent Bonding: Imines are susceptible to nucleophilic attack. An amino group at the active site of the penicillinase reacts with the imine. - Inactivation: This reaction forms a second covalent bond between the enzyme and the inhibitor. This covalent attack permanently inactivates the penicillinase, effectively wiping out the bacterial resistance. - Classification: This type of drug is known as a mechanism-based "suicide inhibitor."