Metabolic handling of ingested fluoride Absorption, soft-tissue distribution, hard tissue uptake, and excretion Objectives: DENT 5302 TOPICS IN DENTAL BIOCHEMISTRY 9 April 2007

Outline Overview of fluoride metabolism Factors affecting fluoride absorption Distribution of fluoride in calcified tissues Soft tissue distribution of fluoride Renal excretion of fluoride Fluoride in saliva

Soluble fluoride compounds: NaF, HF, Na 2 PO 3 F Less soluble compounds: CaF 2, MgF 2, AlF 3 F-F- Low pH (<3.5) e.g., stomach: More as undissociated form HF pH > 3.45 e.g., blood, saliva, tissue fluid: ionized form F - dominates Fluoride ion is important for biological effects H + + F - HF ; pKa = 3.45 pH = pK a + log [A - ] or [HA] pH - pK a = log [A - ] [HA] At pH 2.45 log [F - ] = -1 ; [HF] [F - ] = 1 [HF] 10 At pH 6.45 log [F - ] = 3 ; [HF] [F - ] = 1000 [HF] 1 Diffusibility of HF explains physiological behavior of fluoride

PLASMA (Central compartment) SOFT TISSUES URINE ~ 50% in 24 hrs HARD TISSUES LUNG GI TRACT FECES SWEAT FLUORIDE ~ 50 %Steady state Fluoride metabolism

Ingestion How fast is the absorption and distribution? Peak plasma level < 30 min to an hour Rapidly declining Bone uptake & Urinary excretion Return to normal 3-6 hours (If ingesting small amount) Absorption

1.NaF tablet, fasting stomach 2.NaF tablet + glass of milk 3.NaF tablet + calcium-rich breakfast 4.Intravenous injection (100% bioavailability) Guess this….. Absorption ~ 100 % Absorption ~ 70 % Absorption ~ 60 % In the presence of Al 3+, Ca 2+, Mg 2+ Less absorption of fluoride Increased fecal excretion Ekstrand J et al. Eur J Clin Pharm 1979; 16:211-5 P.O. fasting P.O. milk P.O. breakfast IV Subject received 3 mg fluoride: hour What factors affect F absorption?

Higher acidity of stomach content More fluoride absorbed What factors affect F absorption? Pentagastrin: Stimulates gastric acid secretionBioavailability of F = 97% Cimetidine: Inhibits gastric acid secretionBioavailability of F = 66% Fluoride is absorbed as HF Uncharged molecule (HF) readily passes through biological membrane HF dominates at low pH Why? 40% of oral dose of fluoride is absorbed from the stomach AUC = cumulative plasma F level Pentagastrin Cimetidine

NaF or SnF 2 have bioavailability close to 100% Na 2 PO 3 F has less bioavailability Abrasive may bind fluoride (reduce absorption) Fluoride toothpastes x Fluoride from most dental products is almost completely absorbed when swallowed!! APF (acidulated phosphate fluoride) gel Acidic well absorbed Remains on tooth surface 12 hrs Plasma F concentration ~ 1-2 mg fluoride tablet Fluoride varnish

Plasma = central compartment for fluoride Fluoride in Plasma Enter Distribution Elimination 0.2 ppm F 1.2 ppm F 9.6 ppm F Plasma F of subjects from areas with different water F level Plasma F depend on: F intake Distribution Bone & tissues Clearance Excretion in urine Ekstrand J. Caries Res 1978:12:123-7

T/P = Tissue-water-to-plasma-water ratio Administer (IV) radioisotope fluoride ( 18 F) Determine T/P at various times until the level equilibrates (steady-state) Fluoride is distributed from plasma to all tissues and organs How to study tissue distribution? Inulin (extracellular markers): T/P = T/P > 0.4 = agent can penetrate cells. T/P >1 = agent can accumulate in the tissue Distribution

T/P Brain (blood-brain barrier) Adipose tissue Heart Salivary gland Lung Liver Kidney Inulin (extracellular markers): T/P = T/P = Fluoride is able to penetrate cells but not accumulate intracellularly Tissue Distribution of Fluoride

Uptake of 18 F by the skeleton 4 min after IV injection in laboratory mouse Distribution of fluoride in calcified tissues F - from plasma enters hydration shell Exchanges with OH -, CO 3 2-, F - (apatite crystal surface) Migrates into the crystal interior (slow) Almost 50% of absorbed fluoride is taken up by the calcified tissues Ion-exchange process:

Retention of fluoride in calcified tissues Fluoride in calcified tissues is not irreversibly bound and can be released by ion-exchange or normal remodeling process 80 days: F retention ~ 90% 2 years old: F retention ~ 60% Young animals (& human): High portion of fluoride is deposited in the skeleton F retention ~ 50% Puppies Adults in growing dogs

Kidney is the major route of fluoride excretion Adults: 40-60% of ingested fluoride Children: Excrete a smaller % of ingested fluoride Reabsorb from renal tubules Glomerular filtration Fluoride in plasma Excrete in urine Amount of excreted fluoride vs time after ingesting 30% 60% Excretion Renal clearance of fluoride

Later: Different diuretics have different effect on renal clearance of F. Early study: F Renal clearance increases with urinary flow rate. F excretion: Acetazolamide >>> Furosemide Acetazolamide increases HCO 3 - pH increases Acetazolamide Furosemide Urinary flow rate (  l/min) F clearance

Does Urinary pH or flow rate determine F clearance? Period 1-8: Mannitol diuresis Flow rate ; Urinary pH ; F clearance Period 10-12: Diamox + bicarbonate Flow rate ; Urinary pH ; F clearance Primarily related to urinary pH Secondarily related to urinary flow rate Some diuretics (e.g., mannitol, saline) increase F clearance because the tubular fluid is diluted, thus pH increases. Separate urinary flow rate and urinary pH Conclusion: Tubular reabsorption of fluoride

Capillary HF F - H+H+ H + + F - Acid urine Low urinary (tubular fluid) pH: More HFmore diffusionmore reabsorb Less F - less remainless excrete Tubular reabsorption of F occurs by the diffusion of HF (not F - ) HF can permeate lipid barriers F - is charged and has large hydrated radius incapable of permeating the tubular epithelium Alkaline urine High tubular fluid pH: Less HFless diffusionless reabsorb More F - more remainmore excrete F - HF H+H+ H + + F - Alkaline urine How does pH affect the renal handling of F?

Composition of diet Certain drugs Metabolic diseases Vegetarian diet more alkaline urine more fluoride excreted To promote the renal excretion of fluoride by increasing urinary flow rate (diuresis) (sometimes recommended for acute fluoride poisoning) Why is urinary F excretion important? Acute fluoride poisoning Effective only if urinary pH increases Factors that influence urinary pH:

Fluoride in Feces: unabsorbed fluoride < 10% ingested F Less F absorption if diet high in Mg 2+, Al 3+, Ca 2+ Other routes of fluoride excretion Fluoride concentration ~ 20% of plasma. High end sweat excretion ~ 5% ingested F Tropical climate + prolonged exercise ~ 0.1 mg Compare to ~ 2 mg uptake from diet ~1 mg excreted by urine Feces Sweat

Fluoride in Saliva Duct secretion (systemic, endogeneous) ~ ppm, 30% less than serum F Saliva F-concentration Whole saliva: Duct secretion + exogenous F F-concentration in saliva (1) after toothbrushing (3) chewing F tablet (6) F mouthrinse (7) APF (8) 2% NaF

Recommended references 1.Ekstrand J, Fejerskov O, Silverstone LM (Eds). Fluoride in Dentistry. Copenhagen: Munksgaard Chapters 3 & 7. 2.Ekstrand J, Spak C-J. Vogel G. Pharmacokinetics of fluoride in man and its clinical relevance. J Dent Res 1990;69: Whitford GM. The physiological and toxicological characteristics of fluoride. J Dent Res 1990;69: Whitford GM. Intake and metabolism of fluoride. Adv Dent Res 1994;8: Whitford GM. The Metabolism and Toxicity of Fluoride. 2nd Ed. Monographs in Oral Science Vol 16. Chapters I – IV.