Antitussive Action of Morphine (Opioids)

Mechanism of Action

• Lippincott Pharmacology: "Both morphine and codeine have antitussive properties and cause cough suppression by direct depression of the medullary cough reflex."
• Goodman & Gilman's: "Morphine, codeine, and related opioids depress the cough reflex at least in part by a direct effect on a cough center in the medulla."

Key Features

Receptors Involved

• Stereoisomers of opioid molecules that have no analgesic activity or addiction liability (e.g., dextromethorphan) still produce antitussive effects.
• The antitussive dose is lower than the analgesic dose for codeine (15 mg antitussive vs. higher doses for analgesia).

Clinically Used Opioid Antitussives

Dissociation from Respiratory Depression

Summary Points for Exams

1. Morphine suppresses cough via direct depression of the medullary cough center.
2. The antitussive receptors are distinct from analgesic receptors.
3. Cough suppression occurs at sub-analgesic doses.
4. There is no obligatory link between antitussive action and respiratory depression.
5. Codeine is the prototypical opioid antitussive; dextromethorphan is the most widely used non-addictive alternative.
6. The protective glottal reflex is preserved during opioid-induced cough suppression.

Antitussive Action of Morphine - Full Reference Notes

The Normal Cough Reflex (What Morphine Suppresses)

How Morphine Suppresses Cough

"Morphine, codeine, and related opioids depress the cough reflex at least in part by a direct effect on a cough center in the medulla; this cough suppression can be achieved without altering the protective glottal function."

"Both morphine and codeine have antitussive properties and cause cough suppression by direct depression of the medullary cough reflex. In general, cough suppression does not correlate closely with analgesic and respiratory depressant properties of opioid drugs. The receptors involved in the antitussive action appear to be different from those involved in analgesia."

Key Pharmacological Dissociations

ANALGESIC ACTION          ≠          ANTITUSSIVE ACTION
  μ-opioid receptors                 Different opioid receptors
  Higher doses needed                Sub-analgesic doses sufficient
  Addiction liability                Stereoisomers without addiction work

RESPIRATORY DEPRESSION    ≠          ANTITUSSIVE ACTION
  Obligatory with high doses         NO obligatory link
  Dextromethorphan: antitussive      but NO respiratory depression

Clinically Used Opioid Antitussives

"The opioid analgesics are among the most effective drugs available for the suppression of cough. This effect is often achieved at doses below those necessary to produce analgesia... The antitussive effect is also produced by stereoisomers of opioid molecules that are devoid of analgesic effects and addiction liability."

Summary - Exam High-Yield Points

1. Site: Direct depression of medullary cough center (NTS region)
2. Dose: Below analgesic dose (codeine 15 mg antitussive vs. higher for pain)
3. Receptors: Distinct from analgesic receptors - proven by stereoisomers
4. Glottal reflex: Preserved (protective function maintained)
5. No obligatory link to respiratory depression
6. Best clinical choice: Codeine (opioid) or dextromethorphan (non-addictive)
7. Caution: Avoid in asthma (morphine releases histamine causing bronchoconstriction)

• Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 472 - Cough section
• Lippincott Illustrated Reviews: Pharmacology, p. 711 - Depression of cough reflex
• Katzung's Basic and Clinical Pharmacology, 16e, p. 896 - Antitussives section
• Harrison's Principles of Internal Medicine, 22e, p. 2216 - Cough mechanism

CANCER - Definition, Etiology, and Pathogenesis

1. DEFINITION

"Cancer is a genetic disorder caused by DNA mutations... these genetic and epigenetic abnormalities alter the expression or function of key genes that regulate fundamental cellular processes, such as growth, survival, and senescence."

• Robbins & Kumar Basic Pathology

Key Distinctions

Classification of Cancers

2. ETIOLOGICAL FACTORS

A. Chemical Carcinogens

B. Radiation Carcinogenesis

• Ionizing radiation (X-rays, gamma rays, alpha/beta particles): Causes DNA strand breaks and mutations. Linked to leukemia, thyroid, breast, lung, and bone cancers. Classic example: atomic bomb survivors.
• Ultraviolet (UV) radiation: Causes pyrimidine dimers in DNA, especially in skin cells. Linked to squamous cell carcinoma, basal cell carcinoma, and melanoma.

C. Viral and Microbial Oncogenesis

D. Hereditary / Genetic Factors

• Germline mutations in tumor suppressor genes: BRCA1/2 (breast/ovarian cancer), APC (colorectal cancer), RB1 (retinoblastoma), TP53 (Li-Fraumeni syndrome)
• Hereditary cancers follow the "two-hit hypothesis" (Knudson): one mutant allele is inherited, and a second somatic mutation inactivates the remaining normal allele

E. Acquired Predisposing Conditions (Chronic Inflammation)

3. PATHOGENESIS

Step 1: Fundamental Principle - Cancer is a Genetic Disease

"Genetic alterations in cancer cells are heritable, being passed to daughter cells upon cell division; as a result, cells harboring these mutations are subject to Darwinian selection."

• Robbins & Kumar Basic Pathology

Step 2: Key Genetic Mechanisms

Step 3: Carcinogenesis - A Multistep Process

Fig. General scheme for carcinoma development - from normal epithelium to distant metastasis. (Robbins & Kumar Basic Pathology)

Step 4: The Hallmarks of Cancer

FIG. 6.15 Eight cancer hallmarks and two enabling factors. (From Hanahan D, Weinberg RA: Hallmarks of Cancer. Cell 144:646, 2011 - as cited in Robbins & Kumar Basic Pathology)

• Genomic instability - mutator phenotype accelerates mutations
• Tumor-promoting inflammation - inflammatory mediators promote all cancer hallmarks

Summary Flow of Cancer Pathogenesis

Normal Cell
    ↓ (Initiating event: mutation by carcinogen/radiation/virus)
Initiated Cell (single mutation in key gene)
    ↓ (Promotion: clonal expansion, additional mutations)
Pre-malignant Lesion (e.g., adenoma, dysplasia)
    ↓ (Progression: accumulation of hallmark mutations)
Cancer (malignant, invasive)
    ↓ (Metastasis: invasion → intravasation → survival → extravasation → colonization)
Metastatic Disease

• Robbins & Kumar Basic Pathology, 10e - Chapter 6: Neoplasia (pp. 213-229) - primary source for all above content
• Thompson & Thompson Genetics and Genomics in Medicine, 9e - Chapter 16: Neoplasia (pp. 361-362)
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Carcinogenesis section

What is Cancer?

The Simple Explanation

The Formal Definition

"Neoplasia means 'new growth,' and neoplastic cells are said to be 'transformed' because they replicate incessantly as a result of resistance to the regulatory influences that control normal cells."

• Robbins & Kumar Basic Pathology

1. Local invasion - it grows into and destroys surrounding normal tissue
2. Metastasis - it spreads to distant organs via blood or lymph vessels

Why Does Cancer Happen? - The Genetic Basis

• Proto-oncogenes (the accelerator): Normal genes that promote cell growth. When mutated, they become oncogenes that are permanently switched ON - like a stuck gas pedal (e.g., RAS, MYC, HER2)
• Tumor suppressor genes (the brake): Normal genes that inhibit growth. When mutated/deleted, the brakes fail (e.g., TP53, RB, BRCA1)

Cancer develops when the accelerator gets stuck AND the brakes are cut - a cell is simultaneously told to grow without limit and told it cannot stop.

How Cancer Develops - The Multistep Process

Normal Cell
    ↓  1st mutation (e.g., in RAS oncogene)
Initiated Cell — slightly abnormal, fast-dividing
    ↓  2nd, 3rd mutations (e.g., TP53 lost, RB lost)
Pre-malignant Lesion — e.g., adenoma, dysplasia
    ↓  Further mutations (angiogenesis, invasion genes)
Invasive Cancer
    ↓  Additional mutations
Metastatic Cancer (spreads to liver, lung, brain, bone)

What Makes a Cancer Cell Different - The 8 Hallmarks

The Hallmarks of Cancer (Hanahan & Weinberg, 2011) - as illustrated in Robbins & Kumar Basic Pathology

• Genomic instability - accelerates mutation rate (mutator phenotype)
• Tumor-promoting inflammation - inflammatory signals fuel all 8 hallmarks

Benign vs. Malignant - Quick Comparison

Types of Cancer

Key Takeaway