USING THE POLARITY CHARACTERISTICS OF A DRUG S FUNCTIONAL GROUPS TO ESTIMATE SOLUBILITY CHARACTERISTICS

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1 USING THE POLARITY CHARACTERISTICS OF A DRUG S FUNCTIONAL GROUPS TO ESTIMATE SOLUBILITY CHARACTERISTICS Lessn Objectives 1. Understand what is meant by lgp; knw the difference between MlgP and ClgP 2. Utilize the plarity f a drug mlecules functinal grups t estimate the water slubility and permeability capacity f that drug mlecule 3. Knw the general limits t lgp fr ability t crss lipid membranes (transitin thrugh varius bilgical cmpartments such as gut and vasculature) 4. Knw the general limits t lgp fr ability t shw sufficient slubility in water 5. Knw hw the limits t membrane permeability can be used t therapeutic advantage, as in the case f Mannitl and Srbitl 6. Understand hw different slubilities are required fr different circumstances, as with the frmatin f prdrug esters f Chlramphenicl with carbxylic acids f differing lipphilicities Lemke Methd Organic Functinal Grups, Intrductin t Medicinal Organic Chemistry, 4 th Ed., T.L. Lemke; Lippenctt, Williams & Wilkens (2003) Fye s Principals f Medicinal Chemistry, 5 th Ed., D.A. Williams & T.L. Lemke; Lippenctt, Williams & Wilkens (2003) a. Fcuses n thse functinalities that can cntribute t increased aqueus slubility and assigns a carbn carry alng value i. This is t say Lemke s view fcuses n slubility decreasing with increased mlecular size and assesses functinal grup cntent t determine whether the influence f increased size (increased Van der Waals frces, decreased entrpy f water) has been ffset b. Inizatin (full dissciatin: strng acid with strng r weak base, strng base with strng r weak acid) cntributes a whpping carbn t the estimatin i. Large variatin in the number f carbn s carried shws the difficulty f bth appraches when accurately determining inizatin influence

2 2. lgp a. Als sums functinal grups, but includes values fr bth lipphilic and hydrphilic functinalities b. Greater the number, the lwer the slubility, since P ctanl/water partitin rati; i.e. the amunt f drug mlecule that disslves in ctanl divided by the amunt that can be disslved in water c. USP defines slubility as > 3.3%, r lgp +.5

3 Aside fr the values listed fr the nitr grups, the p values in the table abve weigh the values fr varius systems (such as aliphatic vs. armatic). This makes the apprximatin rather crude as the ability fr charge t disperse is very system dependant. Ntice the large psitive value fr intramlecular hydrgen bnding, IMHB =.65, which reflects the imprtance f hydrgen bnding t water fr slubilizatin.

5 Interpretatin f lgp Values Limits t Slubility Characteristics Bilgical membranes behave as if lipids (plar heads being slvated) As such, bilgical membranes act as barriers t highly plar species (a fully inic species being the upper limit t plarity with regard t individual functinal grups) In rder fr a drug t absrb by the ral rute, it must have sufficient hydrphilic character t disslve sparingly (again nte these are equilibrium prcesses). In rder fr a drug t absrb by the ral rute, it must have sufficient lipphilic character t crss bilgical membranes If drug absrptin is a balance f lipid and water slubility, let s ask the bundary questins

6 Hw much water slubility is t much? Hw much lipid slubility is t much? Perusing lists f physicchemical prperties fr therapeutic agents, we find (tssing ut values that are likely t be carrier mediated) typical lgp values d nt exceed -4. Sme examples are Mannitl (-4.67), Srbitl (-4.67), Zanamivir (- 3.75), and Tbramycin (-3.44) Mannitl des nt crss the bld brain barrier in fact, it des nt crss any barrier and must be administered IV. It is, hwever, freely filtered by the kidney; this fact cupled t its high smtic tendency makes Mannitl a useful smtic diuretic Srbitl is administered rally, but fr use as a laxative (bear in mind it is used as a sweetener in sme sugarless candies) Fr Tbramycin the stry is the same, IV rute r available by inhalatin fr CF patients with Pneumnia (the primary use fr this aminglycside antibitic) Tbramycin is nt absrbed rally Cnsidering the structural similarity between the aminglycsides (e.g. s Gentamicin, Kanamycin) wuld yu expect any f these cmpunds t be available rally? Ntice that Tbramycin is inizable, but that nly makes the cmpund mre plar we are cnsidering the plarity f uninized species t see if they (1) are water sluble (2) have the ptential fr crssing bilgical membranes Zanamivir (Relenza) is available by inhalatin fr influenza A & B

7 Oral biavailability = 1-5% What is a ptential plus f this limited ability t crss membranes? (Hint: Tbramycin is at times administered intrathecally r intraventricularly fr CNS infectins) Well, let s try smething in the lgp = -2 t -1 range Ganciclvir (-2.07) Available rally fr CMV infectin but is prly biavailable Biavailability: 5% fasting, 6-9% with meal, 22% with high fat meal Nucleside analg apparently accunts fr high presence in CSF, ca. 40% f plasma levels Oxytetracycline (-1.22) Gd ral availability Tetracyclines still have widespread use, thugh there are many resistant micrbes ut there

8 Ceftriaxne (-1.76) 3 rd generatin cephalsprin Nt absrbed rally, used fr serius gram neg. infectins Des penetrate int CSF (useful fr meningitis) and crsses the placenta Ceftibuten (-1.06) Als third generatin cephalsprin, with similar prfile t ceftriaxne Rapidly absrbed as capsule r ral suspensin As may be seen by cmparing Ceftrixne t Gancyclvir, the crrelatin f lgp t ral biavailability is nly an apprximatin. T emphasize this pint, 2 ther members f this class are Ceftaxime (-.31) which is nt absrbed rally and nly available as an IV injectin, and Cefixime (-.51) which is available rally, thugh absrptin is slw and incmplete. On the ther end f the spectrum we must ask the questin hw much lipphilicity is t much

9 The majr cncern fr ral absrptin is disslutin, since undisslved drug mlecules are nt absrbed We d have sme cntrl ver water slubility in mst cases (ca. 90% f drugs have an inizable functinality), since we have the capacity t frm inized mlecules r salts which vastly imprve the ability t interact with water in a mst favrable fashin. Further, the GI tract varies in ph frm 1 8, and s drugs will ptimally disslve accrding t their ph. Nte that the lgp f Phenbarbital is 1.71

10 Ntice als the y-axis is mg/100min/cm 2. This is t say that surface area f the slid drug surface is imprtant fr disslutin. If we cnsider the abve in terms f absrptin f a drug, ultimately int the bld stream, we shuld be able t see frm the abve that we are balancing disslutin and slubility with membrane permeability fr slid ral dsing. Frm a bipharmaceutics standpint the bilgical absrbing surface is als crucial, s the upper small intestine ends up absrbing a large amunt f drug agents. Again, tssing ut cmpunds that are likely t be carrier mediated (by apprximatin t endgenus cmpunds) we find therapeutic agents generally fall belw 9 2 examples are Amidarne (8.59) and Halfantrine (8.86) Amidarne (8.59) Antiarrhythmic used fr life threatening ventricular arrhythmias Slw and incmplete biavailability frm ral dsing (generally ca. 50%) Peak plasma in 3-7 hrs Steady state plasma levels nt reached until 1-5 mnths f daily therapy Onset f actin 2-3 mnths Huge vlume f distributin 70 L/kg, Huge eliminatin half-life with persistence f side effects

11 Halfantrine (8.86) Synthetic antimalarial related t meflquine and quinine Absrptin is pr and widely varied n an empty stmach but is increased ca. 10x with a high fat meal Why? It shuld be nted that with the abve examples that increased biavailability with fd are cntrary t the usual case f decreased absrptin (lwer AUC) r decreased Cp max Let s lk at a few cmpunds that have mre usual values (ntice that the barbituric acid cre structure helps t cntrl fr sme f the anmalus differences that might be experienced when cmparing highly varied structures f differing lgp values) Again, aside frm erratic absrptin with high lgp cmpunds, they als tend t sequester int fatty tissues, thus resulting in very high vlumes f distributin and lng half lives. Just s yu knw (believe me, yu will cmmit this relatinship t memry eventually but nt fr this test) t 1/2 = (.7 x V D )/Cl, where the systemic clearance, Cl, is the fractin f the distributin vlume cleared f drug per unit time (fr drugs which are cleared by metablism in the liver, the clearance cannt exceed the hepatic bld flw f 1.25L/min) The fllwing table and graph prvides evidence fr absrptin fllwing lgp acrss a series f cmpunds (with similar pk a s)

12 Nte the abve is fr chlrfrm/water as the partitining system. The values fr ctanl/water lgp are Secbarbital 2.33 Phenbarbital 1.71 The relative ranking is the same

13 The abve crrelatin shuld help drive hme the idea that drug absrptin is a balancing act between aqueus slubility and membrane permeability. In fact, this is the real pwer f lgp determinatins, since we are referring t a partitining between lipid and water that relates well t what we are trying t accmplish achieve a therapeutic cncentratin f drug in plasma. While lgp is reflective f water slubility, it is a better indicatin f this critical partitining relatinship. The fllwing shuld clarify The greater the plarity f the drug mlecule, the greater the slubility in the highly plar aqueus envirnment (like disslves like). Plarity is described by lgp values, which expresses the rati f partitining between ctanl and water in expnential terms Since the lgp partitin rati is defined as ctanl ver water lgp = (lg) the higher the number, the greater the lipid slubility, r lipphilicity, r hydrphbicity. Cnversely, the higher the negative value, the greater the aqueus slubility, r hydrphilicity, r lypphbicity This is a lg scale. MlgP = 0 indicates equal partitining between ctanl and water. A MlgP = 2 between 2 structures indicates a 100x greater partitining int ctanl fr the secnd structure. Similarly, MlgP = -2 indicates a 100x greater partitining int water. Partitining is reflective* f H 2 O slubility. As an example, let s cnsider an apprximatin using ClgP values (ACD Slaris V4.67) between Celecxib and Rfecxib Celecxib, ClgP = 3.01 H 2 0 slubility =.007 mg/ml 1-ctanl slubility = 7.87 mg/ml 7.87/.007 = 1124, MlgP = 3.05 Rfecxib ClgP = 1.63 H 2 0 slubility =.009 mg/ml 1-ctanl slubility =.117 mg/ml.117/.009 = 13, Mlg13 = /13 = 86.5, Mlg86.5 = 1.94 = mg/ml =.0007% (w/v%) slubility.009 mg/ml =.0009% (w/v%) slubility.009/.007 = 1.28, Mlg1.28 =.107 Clearly.107 des nt crrespnd well t the difference in lgp values (ntice that the slight increase in water slubility fr Rfecxib is accmpanied by a tremendus lss in ctanl slubility)

14 Returning t slubility cnsideratins, we must als cnsider hw we wish t be able t deliver a drug when cnsidering ptimal slubility characteristics Let s use chlramphenicl (lgp = 1.02) as an example Brad spectrum antibitic with reasnable penetratin int the CNS (imprtant as an alternative treatment fr Meningitis) Reserved fr treatment f serius infectins resistant t penicillin G and ampicillin wing t serius txicities (bld dyscrasias, bne marrw depressin) LgP <.5 (3.3% H 2 O slubility) is generally taken t mean water sluble Chlramphenicl LgP >.5, but still enugh slubility fr bitter taste t cme thrugh enter palmitate ester as prdrug

15 Hwever, Chlramphenicl is als used in settings where IV administratin is imprtant. Here, we wuld like t increase the water slubility such that there is n chance f precipitating ut a lipphilic species &/r decrease the amunt f material that needs t be injected

Lecture 9 PCL201 Drug Distribution

Lecture 9 PCL201 Drug Distributin Where d drugs distribute? Drug distributin (and ptentially cncentratin) will depend n bld flw and the physichemical prperties f the chemical Lipid and water slubility