I n t h e foregoing, t h e detoxication aspects of drug metabolism were discussed.
Now, attention will be paid to bio-activation. Bio-activation means a change in t h e chemical properties of t h e drug, so t h a t new pharmacological actions are introduced into its spectrum of activity. This m a y be accompanied by t h e loss of its original bio-tropic properties.
One of t h e bio-activations first recognized as such was t h a t of Prontosil R u b r u m (4'-sulfamyl-2,4-diaminoazobenzene hydrochloride), t h e compound found by Domagk (56a) t o be effective in t h e protection of mice against bacterial infections. The drug was inactive when tested in vitro. The cause of t h e dif
ference in activity appeared t o be t h e change of t h e drug by t h e mice from an inactive to an active form—a bio-activation (Fig. 7). Prontosil is metabolized t o sulfanilamide as a consequence of t h e reduction of t h e azo configuration in prontosil (196a). A large number of highly effective sulfanilamide derivatives have been developed since then. Sulfanilamide and most of its derivatives are acetylated in t h e body and then become ineffective as far as t h e infection—
therefore, their action upon t h e microorganisms—is concerned. This time, t h e bio-inactivation performed by t h e host is of advantage to t h e parasite.
From t h e various arsenicals used against spirochetal and protozoal infec
tions, t h e arsenoso compounds ( R — A s = 0 ) have a direct parasiticidal effect.
The other arsenicals are only effective after conversion to arsenoso compounds.
Depending on t h e drug used t h e bio-activation is based on a reduction in t h e
78 Ε . J . A RI E NS AND Α. Μ. SIMONIS N H2
Ο c - c — Ο S / / \ \ _ M = M - / / V LN H. «« «*ro inactive V * H„N—('fi x 7 N> - N = Ν — ( x 7 x 2>-s — N H in vivo activated / \ / / v\ 2 " 1 ν w ; : ' PV
Ο ι C - C - Ο Ο - / / \ \ - N O , prontosil rubrum | parathion
reduction bio-activation oxidation
/ / ~ \ - K U
I
v _ / ^ ~ ~ ^ V - ! L v u iw w/ro active \ ^ι ι
f c - c - o 1
-NH2 H2N —( V S - N H - .*x 7 " * in vivo active y \ 1 . ~ ~ "VC PV
sulfanilamide
c—c—o o-^v y—NO,
paraoxon
acetylation bio-inactivation hydrolysis
1 I I
|r . . Ο J C - C — Ο T O
c_clN_ y / VS_NH2 inactive ^ / / Κ 2 ?:« ?u?>o inactive , \ / / \ \
Ο Λ ' Ο C—C—Ο OH HO—Γ X\ - N 02
JV4 acetyl sulfanilamide
FIG. 7. Bio-activation and bio-inactivation.
case of t h e p e n t a v a l e n t acids ( R — A s 03H2) or on a n o x i d a t i o n in case of t h e arseno c o m p o u n d s ( R — A s = A s — R ' ) (62).
T h e r e l a t i o n s between s t r u c t u r e a n d t o x i c i t y in v a r i o u s series of a l i p h a t i c fluorine c o m p o u n d s a r e i n s t r u c t i v e (see T a b l e X ) . T h e d e r i v a t i v e s w i t h a n odd n u m b e r of m e t h y l e n e groups in t h e chain can be d e g r a d e d b y β - o x i d a t i o n
T A B L E X
ALTERNATION IN TOXICITY OF ALIPHATIC-CU-FLUORINE COMPOUNDS0
F(CH2)nCOOH F(CH2)nCHO F(CH2)nCH2OH F(CH2)MCH3
η Odd* Even6 Odd Even Odd Even Odd Even
1 6.6 6 10
2 60 46.5 3 0.65 2 0.9
4 > 100 81 > 100
5 1.35 0.58 1.2 1.7 6 40 > 100 80 35 7 0.64 2 0.6 2.7
8 > 100 53 32 21.7 9 1.5 1.9 1 1.7 10 57 > 4 0 > 1 0 0 15.5 11 1.25 1.5 2.5
A L D60 for mice in mg/kg intraperitoneally. From Pattison (146a).
b Toxicity in mg/kg for odd and even values of n.
Ι , Ι . Β . DRUG T R A N S F E R E N C E : DRUG METABOLISM 79 under formation of t h e highly toxic fluoroacetic acid (see Section I.B.5.2.) The compounds with a n even n u m b e r of methylene groups lead t o t h e relatively nontoxic j8-fluoropropionic acid. The consequences of these bio-transforma
tions are reflected in t h e toxicities summarized in Table X .
An i m p o r t a n t example of a bio-activation b y reduction is found among t h e adrenal cortical steroid hormones a n d t h e synthetic drugs of t h a t group. The antirheumatic steroids, cortisone a n d prednisone, are practically inactive if applied locally, in t h e joints. The corresponding " h y d r o " compounds, hydro
cortisone a n d prednisolone, are highly active if applied by this route. Ad
ministered orally, cortisone a n d prednisone are as effective as t h e hydro compounds, since t h e y are metabolized in t h e liver t o hydrocortisone a n d prednisolone, respectively (see Fig. 8). Consequently, t h e oral route, which ascertains rapid reduction through early action b y t h e liver, is even more effective t h a n will be subcutaneous injection. I n this case, too, bio-activation is followed by an inactivation, partially because of conjugation with glucuronic acid (149).
Progesterone is metabolized mainly t o pregnanediol. This is a highly active pyrogenic steroid (111a). Possibly, t h e rise in body t e m p e r a t u r e during t h e progestational phase of t h e menstrual cycle is due t o this metabolite. Various aspects of steroid metabolism in relation t o steroid structure a n d t h e conse
quences of these bio-transformations are discussed in a thoughtful review b y Berliner (20b).
Some physiologically i m p o r t a n t bio-activations are t h e decarboxylation of dihydroxyphenylalanine (DOPA) t o dopamine, a precursor of norepinephrine a n d epinephrine, of histidine t o histamine, a n d of 5-hydroxytryptophan t o 5-hydroxytryptamine (serotonin). These amino acids can cross t h e blood-brain barrier; t h e amines formed from t h e m cannot.
Deamination practically always results in an inactivation, however, dealkyl-ation of amines often results in active compounds. Most sympathomimetics are active as free amines as well as methylamines. Compare, for instance, norepine
phrine a n d epinephrine, norephedrine a n d ephedrine, a m p h e t a m i n e a n d meth-a m p h e t meth-a m i n e . Demethylmeth-ation of t h e meth-amino group in morphine meth-and other narcotics results in a decrease of activity (15,121,133). Dealkylation of ring-bound alkoxy groups, which results in an unmasking of phenolic OH-groups, often leads t o active compounds. Deethylation of phenacetine gives iY-acetyl-p-aminophenol, demethylation of codeine gives morphine, etc. Dealkylations are oxidative processes.
Other examples of bio-activation are t h e introduction of phenolic OH-groups a n d t h e oxidation of phosphorothionates among t h e insecticides. P a r a t h i o n , an acetylcholinesterase-inhibitor, inactive in vitro b u t active in vivo, is oxidized by t h e liver t o paraoxon, which is active in vitro as well as in vivo—a bio-activa
tion. Paraoxon, in its t u r n , is metabolized rapidly by t h e liver a n d inactivated (143) (Fig. 7). The toxicity of both compounds, therefore, varies with t h e route
80 Ε. J . ARIENS AND A. M. SIMONIS
of administration. A comparison of t h e intramuscular and intraperitoneal route demonstrates t h a t parathion is least toxic given intramuscularly; para-oxon is least toxic given intraperitoneally (93).
A rather complicated t y p e of bio-activation by oxidation is found in the
ug/100ml Plasma —— — A Hydrocortisone after 200 mg hydrocortisone orally
ο c J r U s o n ^0^ }0^ 2 09 cortisone orally 0 m
200
150
100 Η
50
60 120 180 240 minutes
FIG. 8. Concentration of cortisone and hydrocortisone in plasma after oral administra
tion. Note the high concentration of hydrocortisone after administration of cortisone (149).
toxic syndrome caused by ethyl alcohol after Antabus (tetraethylthiuramdi-sulfide), also called Refusal. The toxic symptoms are caused by acetaldehyde, formed from t h e alcohol by oxidation. Acetaldehyde is a normal intermediate in metabolism of alcohol. Normally, however, t h e concentration remains far below t h e toxic level, because of t h e rapid conversion into acetic acid by an aldehyde dehydrogenase in t h e liver. Only after this enzyme is inhibited by Refusal, can a toxic concentration of acetaldehyde be reached in t h e body. The
Ι , Ι . Β . DRUG T R A N S F E R E N C E : DRUG METABOLISM 81 discomfort caused by alcohol after Refusal, makes t h e drug suitable for forcing alcoholics to refuse alcohol (that is, if t h e y do not refuse their Refusal) (97, 146b).
A bio-activation as a result of hydrolysis m a y be expected after administra
tion of esters of various steroid hormones, used as storage-depot preparations (55, 136). The ester group often blocks OH-groups essential to activity. This makes hydrolysis necessary. The variations in activity of the several esters
β - naphthy lamine non- carcinogenic
a h y d r o x y -β - naphthy lamine
carcinogenic
conjugation with glucuronic acid
(liver)
C6H1 0O6
Ο
NH2 h y d r o l y s i s by β -glucuronidase
(urine)
OH
glucuronide non-carcinogenic
α-hydroxy-β - naphthy lamine
carcinogenic FIG. 8. A. Bio-activation, inactivation, and reactivation as sequential steps in metabol
ism of jS-naphthylamine. Boyland (25a).
must be a t t r i b u t e d to t h e variation in the rate of absorption a n d in distribution in t h e tissues, as well as to a variation in t h e r a t e of hydrolysis of t h e esterified steroid (217). Another example of bio-activation by hydrolysis is: acetyl-salicylic acid, readily hydrolyzed by plasma enzymes, liberates free acetyl-salicylic acid. A great variety of esters are susceptible to plasma esterases (123).
While t h e excretion of drugs in urine usually results in an absolute removal, excretion in t h e bile often means t h a t t h e drug is reabsorbed from t h e intestines and enters circulation again. This is known as t h e hepato-enteric circuit. I t sometimes happens t h a t a conjugated drug excreted in t h e bile is split in t h e intestines converting it into its free form. The free drug is t h e n reabsorbed.
This is a special t y p e of bio-activation. I t happens, for instance, with t h e con
jugation products of phenolic estrogens (165).
β-Naphthylamine and various other aromatic amines can cause cancer of t h e
82 Ε. J. ARIENS AND A. M. SIMONIS
bladder, an occupational disease of workers in dye factories (79a). The meta
bolic conversion of these amines to o-hydroxy derivatives seems t o play a role in this carcinogenic action (207a). Metabolic processes possibly also play a p a r t in t h e localization of t h e cancer in t h e bladder (25a). A bio-activation of an originally inactive conjugation product takes place in t h e bladder (Fig. 8A).
The glucuronide mentioned is not the only potential carcinogenic product formed; t h e sulfate of α-hydroxy-j8-naphthylamine can also serve as a pre
cursor. This conjugation product, however, is n o t hydrolyzed b y urinary arylsulfatase. Urine of patients with cancer of t h e bladder contains a relatively high concentration of ^-glucuronidase. This suggests t h a t an action of this enzyme is a cofactor in carcinogenesis in these cases. Glucosaccharo-1,4-lactone, a potent competitive inhibitor of t h e enzyme has been tried for prophylactic t r e a t m e n t (25a, 128a, 171b). Some investigators deny the role of ^-glucuronidase in bladder carcinogenesis (183c).
Conjugation very seldom brings a b o u t bio-activation. The decreased water-solubility of sulfanilamides after acetylation, with an increased risk of crystal-luria is often mentioned as an example of increased toxicity by conjugation.
B y masking t h e anilide group, t h e process becomes one of bio-inactivation.
The damage from crystalluria, a purely mechanical effect, cannot be counted as a pharmacological effect in t h e strict sense.