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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
TECHNICAL REVIEW The Nitroaromatic Group in Drug Design. Pharmacology and Toxicology (for Nonpharmacologists) Michael J. Strauss University of Vermont, Burlington, Vermont 0540 1
The nitro group plays an important role in the action of certain drugs. Although the detailed mechanisms of action of many nitroaromatic drugs are unknown, it is quite clear that several of these drugs are exceedingly valuable materials for the treatment of disease. There is no question that they pose hazards, but so do many other drugs. The risks of marrow depression and cancer are real but small compared to the benefit gained in the treatment of acute illness. Valid questions may be raised when drugs such as tranquilizers, Le., nitrazepam, are used in large quantities for nonacute illnesses which might be just as well treated with drugs not containing the nitro group, Le., oxazepam. Until more is known about the detailed mechanisms of action at the molecular level, such questions cannot be answered. Until they are answered, the risks and the benefits must be carefully considered and the drugs used with caution.
The nitro group is a unique functional group with a diversity of chemical and biological actions. Its very strong electron attracting ability creates localized or regional electron deficient sites within molecules. When such compounds interact with living systems these electrophilic sites may then react with a variety of intra and extracellular biological nucleophiles, i.e., proteins, amino acids, nuclei acids, enzymes, etc., to produce biological changes which can be useful or harmful, depending on the perspective used to judge them. On a molecular level the interaction may be a nucleophilic addition or displacement, an electron transfer involving oxidation and reduction, or merely molecular complexation without formation of a formal covalent bond. The biological changes which result can be deleterious to the organism as a whole, but in many cases toxicity is selective; that is, it can result in poisoning of bacteria, parasites, or tumor cells without harming the host organism or normal cells. Such selective toxicity is the basis for chemotherapy (Albert, 1964). There are numerous drugs which exert their primary pharmacological action because of the presence of an aromatic nitro group. A few of these are shown in Figure 1,along with others in which the presence of a nitro group may only be of secondary importance. The structural types and functions are quite diverse and include antineoplastic, antibiotic, and antiparasitic drugs, as well as tranquilizers, fungicides, insecticides, herbicides, and others. In all cases the nitro group or some other electron attracting functionality is usually necessary for the specific biological effect desired. Before discussing the mechanisms of action 0019-7890/79/1218-0158$01 .OO/O
of several of these drugs (where they are known), it is appropriate to consider the more general types of biological effects caused by nitroaromatic compounds. These include uncoupling of oxidative phosphorylation, formation of methemoglobin, bone marrow depression, and allergic reactions.
Uncoupling of Oxidation and Phosphorylation The generation of adenosine triphosphate (ATP) during the oxidation of foodstuffs by aerobic organisms is the major source of energy in metabolic reactions. The energy stored in the pyrophosphate bonds of ATP is used in numerous coupled reactions to provide the overall driving force for various anabolic sequences. ATP generation (phosphorylation) is coupled to foodstuff oxidation. Reaction I represents food oxidation. It does not occur
SH2 + ' / 2 0 2 foodstuff
+
S + HzO oxidized foodstuff
( 1)
directly, but by a series of reactions involving coenzymes and cytochromes, Figure 2. ATP formation is coupled to this sequence at several places and it is this ATP which is a major source of energy for anabolic metabolism. Interference with this process of ATP formation would obviously disrupt many crucial reactions of metabolism. A number of aromatic nitro compounds are capable of doing this. They act by uncoupling the oxidation chain (horizontal progression of electron transport shown in Figure 2) from the process of phosphorylation (ADP + phosphate ATP). The result is foodstuff oxidation without concomitant formation of vital ATP. A classic example of this effect is the action of 2,4-dinitrophenol. Such uncoupling agents are lipid soluble substances containing an aromatic ring with an acidic ring substituent.
-
In many instances nitro groups are not necessary, but in 2,4-dinitrophenol they act by conferring substantial acidity on the phenolic hydroxyl. The mechanism of uncoupling by this compound is not yet known. Its toxic properties have been well characterized, however. In mammals it 0 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
causes a marked increase in metabolism and body temperature, and at high levels it is fatal. Its use as a weight reducing drug was long ago discontinued. Oxidation of Hemoglobin to Methemoglobin Nitroaromatic compounds are metabolized in several ways. They can be reduced to a variety of intermediate reduction products. Eventually reduction to the corresponding aniline can occur, followed by acylation and excretion. (See Scheme I.) Alternatively nitroaromatics may be ring hydroxylated, conjugated with glucose, and then excreted. A variety of compounds are capable of acting as oxidants in the process of converting Fez+ to Fe3+ in hemoglobin, and nitroaromatics are quite effective in this regard. The
-
6H+ + ArNOz + 6Fe2+(hemoglobin) ArNH, 6Fe3+(methemoglobin) + 2 H 2 0 (11)
+
resulting methemoglobin does not complex with 02,and the oxygen carrying ability of the blood is thus reduced. With large amounts of nitroaromatic the result can be fatal. Formation of carboxyhemoglobin by complexation of carbon monoxide with hemoglobin produces the same physiological effect, tissue deprivation of oxygen. Oxidative metabolic processes are thus disrupted and in the case of severe intoxication, either by CO poisoning or methemoglobin formation, it is not surprising that death can result. Bone Marrow Depression Anemia is commonly associated with a variety of disease states and can result from numerous factors, including vitamin or iron deficiencies, defects in protein synthesis, blood loss, or blood cell destruction. There are a number of chemical compounds which are capable of specifically disrupting the ability of the bone marrow to produce erythrocytes (red blood cells), leukocytes (white blood cells), and platelets, the cellular elements of blood. In such a case, the marrow is unable to produce sufficient cells to replace those normally utilized and a total reduction of these cells in the plasma results (pancytopenia). In most instances, the precursor cells in the marrow also become diminished in number and replaced by fatty tissue (aplasia). Compounds most commonly associated with bone marrow depression are those which are cytotoxic (cellular poisons which disrupt nucleic acid and protein synthesis). These are useful as anticancer drugs if their toxicity is selective enough. Examples are the antimetabolites, analogues of natural substrates which inhibit normal metabolic pathways, and alkylating agents which react with macromolecules (Le., DNA) and thus inhibit cell division. Simple aromatic compounds such as benzene, and especially nitroaromatic compounds, also have the specific ability to cause bone marrow depression. These are called myelosuppressive agents (as are many of the alkylating agents and other cytostatic compounds). The term myelo is a combining form denoting relationship to the bone marrow. Marrow depression can lead to a combined pancytopenia and acellular marrow, a classic example of aplastic anemia. This ultimate result is extremely serious indeed, for the outcome is often fatal. It is not an uncommon result of exposure of laboratory personnel to excessive amounts of aromatic and nitroaromatic compounds. The manufacture of nitroaromatic explosives and aromatic amine containing dyes can pose a serious hazard to workers in these facilities if adequate precautions are not taken.
7.IOH
I1 :iH b C I1t
,
1 ' I e t r (1 n 1 d a 1 0 1 e (amcbicide)
CH2flH C11 l a
I
CtIl
7
159
ranphen ica 1
Nltrazeparn (tranquilizer)
(antibiotic)
Pyrrolnitrin (fungicide)
Par ath
Niridarole
(schistosanicide)
(In5ec t
10"
i c ide )
0"; "H
2 , 4 - d i n i t r o p lhc n o 1 (weed
killer) AIR t 11 1 o p r i n e (immUnOSUppreSSanC,
Jntincoplastrc)
Figure 1. Drugs containing aromatic nitro groups.
Scheme 1 ArNO
Ar-NHOH
, e Ar-NH2
II
1
It
0
A r -NO2
I
I HO-Ar
II
Ar-NH-2-R
excretion
t -NO2
9
glucose -0-Ar-NO2
Allergenicit y Although the immune response is a defense against infection, it also is responsible for allergic reactions to a variety of chemicals. These can in many instances be quite distressing. Nitroaromatic compounds with particular structural features are well known as haptens, Le., substances foreign to the body which can evoke formation of an immunogen, a material capable of initiating the immune response. If the nitro group on an aromatic ring activates another group on the same ring towards nucleophilic displacement, reaction with basic functions on proteins can readily occur to covalently bind the nitroaromatic group. The resulting phenylated protein is an immunogen which can react with induced antibodies to elicit the response. A particularly interesting example of this phenomenon is the differential allergenicity of 1,2,4-and 1,3,5-trinitrobenzene (Venulet and Van Etten, 1970). Only in the former is there a leaving group, nitrite, activated for nucleophilic substitution by ortho and para nitro groups. The 1,3,5 isomer is much less reactive in nucleophilic aromatic substitution reactions, forming only a relatively unstable addition complex (Strauss, 1970). Thus only in the 1,2,4isomer is the rash and dermatitis produced by the allergic reaction (immune response) as common (See Scheme 11.) Interestingly, l-chloro-2,4-dinitrobenzene has been used
160
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 P3P
2H At
(reduced s u b s t r a t e )
t
Phosphate
i I
uncoupling of o x i d a t i v e p h o s p h o r y l a r i o n by dinirrophenol
Figure 2. Electron transport and oxidative phosphorylation. Scheme I1
group was found to be absolutely necessary for activity, and thousands of publications on 5-nitro-2-substituted furans have appeared since the initial discovery. Most of the very active compounds have the general structure 1. h
.u
o,p-activated haDtan
immunogen 0 2
(nitrofurantoin) NOz
no o,p activation
N02-
unstable anionic sigma complex
Scheme I11 H H H2NNCH2C02Et
X
to measure immunocompetence (Rolley et al., 1974) and also to induce cellular immunity within superficial tumors (Frei and Bodely, 1972). In the latter case observation of tumor regression provides strong evidence for the importance of the immune response to tumor restraint (Frei and Bodely, 1972).
Drugs Containing the Nitro Group. Structure, Synthesis, Uses, and Mechanisms of Action Antimicrobials. There are several major types of antimicrobials and antibiotics which contain the nitro group as a necessary prerequisite for activity. A number of these are of clinical importance. Some are natural products and others are synthetic variations on the naturally occurring materials. Only the important drugs now in clinical use will be considered here. The Nitrofurans. In an antibacterial screen of a variety of compounds carried out by Dodd, Stillman, and their associates in 1944 the significant antibacterial activity of 5-nitro-2-substituted furans was discovered. The nitro
3 (nitrofurazone)
The two nitrofurans of major clinical importance as antimicrobials are nitrofurantoin and nitrofurazone, 2 and 3. Nitrofurazone is easily prepared by nitration of furfural and reaction of the resulting nitroaldehyde with semicarbazide, as shown in reaction 111. Nitrofurantoin is
prepared by reaction of hydrogen cyanide with the cyhydrazinoester 4 to yield the carbamate 5. Cyclization of 5 in strong acid yields the hydantoin 6 which condenses with 5-nitrofurfural to yield 2 (Hayes, 1952). (See Scheme 111.) Both 2 and 3 are active against a wide variety of gram positive and gram negative organisms. They are bacteriostatic (inhibit bacterial growth) but can be bacteriocidal (kill bacteria) at high concentrations. The detailed mechanism of action of the nitrofurans is unknown, but they probably interfere with enzymatic processes essential to bacterial growth. It has been suggested that since, for a series of 2-substituted 5-nitrofurans, the antibacterial activities correlate roughly with the energy of the lowest vacant MO (and also with the reduction potential, i.e., the most easily reduced are most active) reduction of the nitro group is an important in vivo reaction necessary for activity (Hirano et al., 1967). A variety of metabolic systems are inhibited by nitrofurazone, mainly because of its ability to act as an electron acceptor under the experimental conditions (Baker et al., 1970). It
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
I61
Scheme V
Scheme IV
O>c-”z
acid f o r m
-0,N
A o X C H - N = N
PH2
basic f o r m
-
complexation, a d d i t i o n or displacement product
O2N
0 > = ~ ~
Scheme VI OH NO?
H+NH~
has also been suggested that NADH acts as a coenzyme in this reduction process (reaction IV), and a stable 1:l complex between 6 and NADH has been studied (Hirano et al., 1967). This has led to speculation that nitrofurans
H+NH,
CH20H
CH20H
separated by fraction crystallization
H NHAc D i - A v C H z OAc
-0Ac
nitration
H NHAc
H
ribose /
AcO H
H NH2
?I
/I
-
CICHpCOCH3
chloramphenicol
OH H r e s o l v e d by t r a c t i o n a l crystallization of the c a m o h o r s j l f o n a t e sal1
ribose NADH COMPLEX ( f o l l o w e d by r e d u c t i o n ? )
(Iv)
possibly act by complexing or reacting with an intermediate in the electron transport sequence, Figure 2. Thus, toxicity may be initiated by irreversible reduction of the nitrofuran. If so, the action would appear to be quite selective since these compounds have a relatively low systemic toxicity. Nitrofurazone is used as a broad spectrum-topical-antibacterial drug in treating skin infections and minor wounds. Interestingly, as with other topical preparations, sensitization can result in as many as 2% of the patients treated. This sensitization may result from the nitrofuran acting as a hapten by reaction with protein thiol or amino functions. (See Scheme IV.) Nitrofurantoin has found widest use in treating bacterial infections of the urinary tract since many common infections which localize there are susceptible to it. It is also
well absorbed from the gastrointestinal tract when taken orally and rapidly concentrates in the urine. Interestingly, its activity is higher in acidic urine. This suggests that either it is concentrated there more effectively when in the acidic form or that nucleophilic addition to the ring might destroy its activity. (See Scheme V.) Nitrofurantoin produces a variety of side effects. The more serious of these include allergic reactions and blood disorders. These are understandable in light of the general effects of nitroaromatics discussed in the introduction. Chloramphenicol. Chloramphenicol is an antibiotic isolated from filtrates of liquid cultures of streptomycetes found in a Venezuelan soil sample. It has marked effectiveness against many gram-positive and gram-negative bacteria. A number of different synthetic routes to chloramphenicol are available and several are probably now in commercial use. One of the early syntheses from benzaldehyde and nitroethanol is outlined in Scheme VI.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 0
fatal. This toxicity is not dose related and the incidence of it is very low (one in 40000). The risk does mean that this drug should only be used where other agents would be ineffective, that repeated uses on a single patient should be avoided, and that blood counts of patients in therapy is a necessity.
II
OH c hlor a rnp hen i c o l
Interestingly, replacement of the nitro group in chloramphenicol with other functionality (electron withdrawing
cetophenicol U
II
Some Clinically Useful Antiparasitic Drugs Niridazole. There are a variety of therapeutic agents used to treat infections caused by parasitic worms, and nitroheterocycles play a prominent role in this regard. Such agents are called anthelmintics. It has been estimated that one-third of the human race suffers from helminth infections, including an estimated 40 million Americans. After a very extensive program of synthesis and screening of heterocyclic nitro compounds for antibacterial and antiparasitic activity (Shmidt and Wilhelm, 1966), a thiazolylimidazolidinone called niridazole was selected as
NHCCHC12
0
-N
c H 3 0 z s ~ ~ H b H c H 2 0 H i
OH niridazole
thiamphenicol less a c t i v e d e r i v a t i v e s o f chloramphenicol
or donating) results in reduction or loss of activity (Bambas et al., 1955; Cutler et al., 1952; Evans et al., 1954; Franklin et al., 1954; Morris and Smith, 1954). This emphasizes the importance of the electronegative nitro group as a prerequisite for such activity. Even moving the nitro group to the meta position diminishes antibacterial action considerably. These changes in activity with changes in structure may mean that the acidity of the benzylic proton is of some importance. (See reaction V.) Of all the other OH O
~
-
-0zN-l
N
-
H
\R
(v)
~
aromatic analogues tried (pyridyl, furyl, thienyl, naphthyl, etc.) only the nitrothienyl derivative was active, but less so than chloramphenicol (Perlman, 1970). r
U
II
NHCCHCIz O2N
CHCHCH20 H
I
OH
Chloramphenicol exerts its biological action by inhibiting protein synthesis in the bacterial cell. The effect is stereospecific and occurs only with one of the four stereoisomers. The activity of peptidyl transferase which catalyzes peptide bond formation is suppressed. The detailed mechanism of this interaction is not known, but it is reasonably selective and chloramphenicol is a valuable antibiotic for treating infections in which it is the most active agent (typhoid fever, bacterial meningitis, as well as severe salmonella and staphylococcalinfections resistant to penicillin). Unfortunately, chloramphenicol causes hypersensitivity reactions in many individuals, and the most important of these is on the bone marrow. Except for the anticancer agents, of all the drugs which cause a decrease in red cells, white cells, and platelets, chloramphenicol is among the most common serious offenders. Resulting aplasia of the marrow can occur, and when complete is almost always
the most effective antischistosomal drug. Schistosomes are blood flukes which, unlike other similar parasitic worms, exist in both male and female forms, with digestive tracts, functional organs, a nervous system, and a complex reproductive process. Niridazole is rapidly taken up by this parasite when it is administered orally and it causes rapid inhibition of egg production in the female worm. Both male and female worms decrease in size, and the females are then destroyed in the liver by white blood cells. This therapeutic action may result from the known ability of niridazole to decrease the worms' uptake of host blood glucose (perhaps due to inhibition of parasite hexokinase) and to reduce parasite glycogen stores by inhibition of parasite glycogen phos~ ~ phorylase activity. Metronidazole. Another parasitic infection of considerable importance is that resulting from a particular species of ameba, Entamoeba histolytica, a one-celled animal with a very complex five-stage life cycle. E. histolytica lives and multiplies in the human intestine. This particular parasite obtains nourishment by destruction of the intestinal wall, leading to the classic symptom of amebic dysentery. Infection can also become extraintestinal and result in abscess of the liver. In Africa about 83 deaths per 100000 population are caused by this parasitic infection and it is estimated that 5% of the population of the United States is infected. The discovery of the antibiotic azomycin (2-nitro-
azornycin
metronidazole \ f lagyl)
imidazole) in the 1950's and the demonstration of its antiparasitic activity led to an extensive synthetic and screening program involving many nitroheterocycles. One of the most interesting of these drugs was metronidazole (Powell, 1971). It is a relatively new drug which can cure amebic infections as well as trichomoniasis. This latter disease is caused by trichomanads, parasites which are found on the surface of the intestinal wall or urinary tract. The introduction of metronidazole for the treatment of all forms of amebiasis has been characterized as the most
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 163 neur0tra"smltter acetylcholine
acetvkholinesrerase
parathion
h y d m l y t i c cleavage
Figure 3. Irreversible inhibition of acetylcholinesterase.
significant advance in the treatment of amebiasis in recent years (Rollo, 1975). This agent is active against all stages of infection and is the drug of choice for chemotherapy. The mechanism of action of metronidazole is not yet well established, but its selective toxicity is astounding. Large doses appear to have few effects in man which are sufficiently severe to necessitate discontinuing the treatment. The nitro group is necessary for activity. Unfortunately, very recent evidence suggests that metronidazole may be mutagenic (The Medical Letter, 1975), and its present use in large numbers of people is a cause for concern. The possibility for long-term carcinogenesis must be considered and this may soon throw a shadow over this useful drug (vide infra). Nifurtimox. Another important protozoan infection affecting man is trypanosomiasis, an extremely dangerous and many times fatal disease caused by parasites of the genus Trypanosoma. Trypanosomiasis is spread by intermediate hosts such as insects and approximately one-quarter of the land mass of Africa cannot be used to raise domestic animals because of the endemic nature of the parasite in this region. The diseases of man caused by Trypanosoma include various types of sleeping sickness as well as Chagas disease. The latter (caused by the specific organism T. cruzi) affects more than 7 million people from Chile and Argentina to Mexico, and approximately l/lo of those infected eventually die from the disease. The clinical manifestations include fever, rash, and fluid accumulation in the face and body. Heart disease is a common late manifestation which can affect up to 10% of the population in endemic areas. Until recently there was no specific treatment for Chagas disease. Fortunately the nitrofuran derivative Nifurtimox
CH 3
n i f u r t irnox
was prepared and shown to be an effective drug for the treatment of acute and chronic Chagas infection (Bock et al., 1972). This drug is very rapidly metabolized and as yet its mechanism of action is unknown. Again, as with the other nitroheterocyclic antibiotics and antiparasitics, the nitro group is absolutely essential for activity.
The Nitroaromatic Moiety In Organophosphate Insecticides. A Toxicological Problem There are a large number of important organophosphorus insecticides of general formula 7. UnfortuX
-
C H , - ~ - O ~ " '
7
1
X=OorS
nately, these are not selective in their toxicity and are thus important from a toxicological viewpoint in man. A particular example of these potent insecticides is parathion (reaction VI). The nitro group is important in these compounds since, for example, resonance contributor b to the ground state of parathion serves to activate electrophilic phosphorus for nucleophilic attack. n
a
parathion
b (VI)
The mechanism of action of the nitroaromatic organophosphorus insecticides is well established. They covalently bind with the active site of the enzyme acetylcholinesterase. This enzyme is responsible for the hydrolysis of acetylcholine, a neurotransmitter which mediates nerve impulses. When acetylcholinesterase is blocked by the insecticide, levels of acetylcholine build up. This results in a variety of symptoms and, in man, the syndrome associated with organophosphate poisoning is well established. The mechanism by which these enzymes act is shown in Figure 3. The nitro group activates the phosphorus toward initial attack by a serine hydroxyl at the active site of the enzyme. Once covalently bound, the nitro group also stabilizes the p-nitrophenylate leaving group which is displaced upon formation of the stable blocked phosphorylated enzyme. Exposure only to the vapors of a nitroaromatic organophosphorus insecticide will produce watering eyes and nasal discharge, tightness in the chest, and wheezing respiration due to bronchoconstriction and bronchial secretions. Accidental eating of food contaminated with
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
184
Scheme VI1
SH
"2
I
I
adenine
8
9
azathioprine 8 t
-
Nu
S
-
6-mercaptopurine
I NH
6-mercaptopurine
normal purine biosynthesis and is a useful drug in treating certain types of cancer. With the objective of preventing rapid oxidative metabolism of 6-mercaptopurine, to effect possible selective release in tumor tissue, and to sustain its activity in vivo, the masked pro-drugs of 6-mercaptopurine 8 and 9 were prepared (Elion et al., 1962). (See Scheme VII.) The function of the nitro group in these pro-drug derivatives is to activate the phenyl and imidazole rings towards nucleophilic substitution which liberates the parent drug. This occurs in vivo with the sulfhydryl nucleophile-peptide glutathione as well as with amino acids. The masking imidazoyl group is found bound to nitrogen and sulfur in amino and glutathionyl metabolites. Azathioprine has been shown as an effective antineoplastic drug when given orally. Today it is used extensively as an immunosuppressant in organ transplant patients. The dinitrophenyl derivative 9 has shown no significant advantages over the parent drug 6-mercaptopurine. Further investigations of ester derivatives of 9 are now in progress in the form of the second generation pro-drugs 10 (Drawbaugh et al., 1976) (reaction VII). C02R
CO 2H
I
I
I
CHzCHCNHCH 2 C 0 2 H
R
I "
c=o
I
CHzCHzCHCOzH
I
"2
\
f u r t h e r degradation o f glutathione side chain
such compounds produces nausea, vomiting, stomach cramps, and diarrhea. Severe intoxication produces involuntary defecation and urination, a slowing of the heartbeat, low blood pressure, weakness, and paralysis. The latter can be fatal if the respiratory muscles are involved. The final cumulative action in the central nervous system leads to loss of reflexes, coma, complete loss of respiratory drive, and death. The time of death after a single exposure may vary from 5 min to 24 h depending on the amount absorbed and the particular compound involved. It should be quite clear that these insecticides, which are still in use, pose a severe hazard to those not aware of the dangers involved. Farm workers are particularly prone to such exposure. The Nitro Group in Drug Latentiation. Use with an Antimetabolite The term "pro-drug'' was first used by Albert in 1958 to describe chemically modified drugs which undergo biotransformation prior to exhibiting their pharmacological effects (Albert, 1958). He noted that such modification can introduce selectivity into toxicity. The chemical differences between the drug and pro-drug are such that the absorption and distribution properties of the parent may be dramatically altered in the pro-drug. This can lead to selective distribution, possibly diminish toxicity and side effects, and also perhaps provide for a sustained action (Higuchi and Stella, 1975). 6-Mercaptopurine is an antimetabolite closely related to adenine. It acts as a cytotoxic agent by disrupting
Nitroaromatic Herbicides-Plant Toxins It has been known for quite some time that dinitrophenols are quite toxic to plants and are thus effective herbicides. The most effective compounds are those containing nitro groups in the 2,6 and 2,4 positions. Since OH
OH
,
NO2
both 2,4- and 2,6-dinitrophenols are quite effective at uncoupling oxidation from phosphorylation it seems quite reasonable to propose a mechanism of action based on this effect. Recall that one of the requirements of uncoupling agents is an ionizable acidic group. Removal of the acidic hydroxyl by formation of nonionizable derivatives dramatically reduces the herbicidal effectiveness of these compounds (Pianka and Browne, 1967). Interestingly, acetylation enhances the activity (Pianka and Browne, 1967). In this case it is possible that the acetylated derivative is acting as a pro-drug, allowing penetration
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
165
eNo2 Scheme VI11
0
II OCOR
OH
I
NO2
NO2
ethers
carbonates
through a lipoid barrier, followed by hydrolysis back to the parent drug (Scheme VIII). It is also interesting to note that the dinitrophenols are more active than either the mononitro or trinitro analogues. One nitro group may not be sufficient to result in significant ionization (acidity) in vivo, and three may activate the ring for nucleophilic addition both to the free phenol and the anion (Scheme IX). There are several classes of herbicides of the following general structure. These are quite active compounds and
0
0
II OCCH3
qNo2 I
II
OCCH3 I
I
NO2
Scheme IX OH I
OH I
Y
NO2
R = CH,, CH(CH,)C,H,, CH(CH,)C,H,, C(CH,),
their effectiveness has been attributed to the ease with which they form free radicals (Brian, 1955). There is no substantial evidence for a detailed mechanism of action however. Nitroaromatic Compounds as Chemical Carcinogens One of the compounds appearing on the Occupational Safety and Health Administration's list of known carcinogens in man is p-nitrobiphenyl. It is well known that
/\ necessary for herbicidal activity ?
-No2
\ 11 0-
a variety of aromatic primary amines are highly carcinogenic materials and these also appear on the OSHA list. A few are shown below. Since nitroaromatic compounds
&
m
a-naphthylamine H
\~ /
N
"
2
P-naphthylamine
-~
benzidine
N
~
H
-
z
N
p-aminobiphenyl N-N '(NkNH2 H
3-aminotriazole
are many times metabolized by reduction to the corresponding amines, the potential carcinogenicity of aromatic nitro compounds should be quite clear. Present hypotheses regarding the mechanism of carcinogenic action of certain aromatic amines involve activation by N-hydroxylation. The N-hydroxylated amine may then react with specific proteins which are important growth-limiting factors. Reaction with hydroxylated aromatic amine derivatives may diminish the reactivity of these proteins and uncontrolled growth (tumor) may then occur. Carcinogenic aromatic amines can also react with DNA, however, and
the exact mechanisms by which they induce cancer are still unknown (Suss et al., 1973). The real problem comes when the long-term effects of nitroaromatic drugs are considered. If such a drug is given on a largeHscale to vast portions of the population, any ~ evidence of carcinogenicity must be considered in light of the possible latent long-term effects which may be observed. A particular example is the antiparasitic drug metronidazole discussed above. Recent evidence has shown this drug to have mutagenic properties, and as a general rule good mutagens are good carcinogens. The difficulty comes when one has to assess the short term real benefit provided by this drug relative to the long-term possibility of cancer development many years hence. The problem is further complicated since the evidence for mutagenesis is indirect, i.e., not from testing in man. Only in the case of an acute parasitic infection which will run a fatal course is the answer absolutely clear. Literature Cited Albert, A., Nature(London), 182, 421 (1958). Albert, A., "Selective Toxicity", Wiiey, New York, N.Y., 1964. Baker, J. W., Schumacher, I., Roman, D. P., "Antiseptics and Disinfectants", Chapter 26 in A. Burger's "Medicinal Chemistry", Wlley. New York, N.Y., 1970.
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Bambas, L. L., Troutman, H. D., Long, L. M., J . Am. Chsm. Soc., 77, 7 (1955). W,M., Haberkom, A., Herlinger, H., Mayer, K., Peterson, S., Ar.zmim.-forsch, 22, 1564 (1972). Brian, R . C., Chem. Ind. (London), 1965 (1955). Cutler, R. A., Stenger, R. J., Suter, C. M., J. Am. Chsm. Soc.,74, 5475 (1952). Dodd, M. C., Stillman, W. B., J. fharmacol. Exp. Ther., 82, 11 (1944). Drawbaugh, R., Bouffard, C., Strauss, M., J . Med. Chem., 19, 1342 (1976). Elion, G. B., Callahan, S. W., Hltchings, G. H., Rundles, R. W., Laszlo, J.. Cancer Chemother. Rep., 18, 197 (1962). Evans, D. D., Morris, D. S.,Smith, S. D., Tivey, D. J., J . Chem. SOC.,1687 (1954). Franklin, C. S., Morris, D. S.,Smith, S. D., J . Chem. SOC., 1683 (1954). Frei, E., Bodely, G. P., "Principles of Neoplasia", Chapter 319 in Harrison's "Principles of Internal Medicine", 7th ed, McGraw-Hill, New Yo&, N.Y., 1972. Hayes, K. J., US. Patent 2610 181 (1952). Higuchi, T., Stella, V., ACS Symp. Ser., NO. 14, (1975). Hirano, K., Hoshina, S., Okamura, K., Suzuka, I., Bull. Chem. SOC.,Jpn., 40, 2229 (1967). The MedicalLetter, 17 (13), 53 (1975). Morris, D. S.,Smith, S. D.. J . Chem. Soc., 1660 (1954).
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Received for review April 24, 1979 Accepted May 18, 1979
POLYMER COATINGS SECTION Design and Evaluation of a Plug Flow Reactor for Acid Hydrolysis of Cellulose Davld R. Thompson and Hans E. Grethlein" Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755
An isothermal plug flow reactor was developed to study the kinetics of acid hydrolysis of cellulosic substrates. The kinetic parameters in a model which gives the glucose formation from purified cellulose (Solka-Floc),200 mesh, were obtained over the following range of independent variables: temperature from 180 to 240 OC,sulfuric acid concentration from 0.5 to 2.0%, and slurry concentration from 5.0 to 13.5%. It was determined that the glucose formation from newsprint (a mixture of wood and pulp), 65 mesh, can be predicted from the kinetic model developed for Solka-Floc. Since at least 50% of the potential glucose can be obtained at 240 O C , 1 % acid, and 0.22min residence time, the continuous acid hydrolysis of cellulose may be a process of commercial interest.
Introduction The future need to replace petroleum has stimulated interest in the conversion of cellulose to a liquid fuel and chemical feed stocks (Wilke, 1975). Moreover, the possibility for converting cellulose from sources such as municipal refuse, agricultural wastes, and wood wastes may be a way of turning a waste disposal problem into a resource opportunity. Acid hydrolysis of cellulosic substrates followed by fermentation of the sugars to ethanol is one method for accomplishing this conversion. In the past, numerous attempts at hydrolyzing cellulose with low concentrations of sulfuric acid have been tried commercially. For example, Saeman's work in the 1940's led to the Madison Process-a semi-batch process for wood hydrolysis (Saeman, 1945). The economics of the process, however, could not compete with ethanol from petroleum, which was introduced a t that time. In 1967, Proteous (1967) proposed that municipal refuse could be disposed of at a profit to the community if the cellulose in the refuse were converted to glucose through acid hydrolysis and then fermented to ethanol. This 0019-7890/79/12 18-0166$01.OO/O
initiated the work again on acid hydrolysis. Fagan et al. (1971), using transient batch experiments, demonstrated that the kinetic model developed by Saeman could also be used for paper. As a result, flow sheets for a new large-scale continuous process have been prepared (Grethlein, 1978) using a plug flow reactor and the kinetic data on paper hydrolysis. In this type of process analysis one has to make some reasonable but untested assumptions to evaluate the economic potential of acid hydrolysis. The major assumptions which are the focus of this study are: (1)that 50% of the potential glucose is obtained in a plug flow reactor; (2) that the slurry concentration in the range of 10 to 30% can be pumped, heated, mixed with acid, reacted, and quenched in a stable steady-state continuous process; and (3) that the kinetic model developed for dilute slurries is indeed valid for concentrated slurries. In order to obtain reliable kinetic information in the economic region of the design space of temperature, acid concentration, and slurry concentration, a laboratory scale continuous plug flow reactor was developed for acid hydrolysis (Thompson, 1977). Because of the high tem0 1979 American Chemical Society
