Synthesis, antimalarial activity, and molecular modeling of tebuquine analogues.

Tebuquine (5) is a 4-aminoquinoline that is significantly more active than amodiaquine (2) and chloroquine (1) both in vitro and in vivo. We have developed a novel more efficient synthetic route to tebuquine analogues which involves the use of a palladium-catalyzed Suzuki reaction to introduce the 4-chlorophenyl moiety into the 4-hydroxyaniline side chain. Using similar methodology, novel synthetic routes to fluorinated (7a, b) and a dehydroxylated (7c) analogue of tebuquine have also been developed. The novel analogues were subjected to testing against the chloroquine sensitive HB3 strain and the chloroquine resistant K1 strain of Plasmodium falciparum. Tebuquine was the most active compound tested against both strains of Plasmodia. Replacement of the 4-hydroxy function with either fluorine or hydrogen led to a decrease in antimalarial activity. Molecular modeling of the tebuquine analogues alongside amodiaquine and chloroquine reveals that the inter-nitrogen separation in this class of drugs ranges between 9.36 and 9.86 A in their isolated diprotonated form and between 7.52 and 10.21 A in the heme-drug complex. Further modeling studies on the interaction of 4-aminoquinolines with the proposed cellular receptor heme revealed favorable interaction energies for chloroquine, amodiaquine, and tebuquine analogues. Tebuquine, the most potent antimalarial in the series, had the most favorable interaction energy calculated in both the in vacuo and solvent-based simulation studies. Although fluorotebuquine (7a) had a similar interaction energy to tebuquine, this compound had significantly reduced potency when compared with (5). This disparity is possibly the result of the reduced cellular accumulation (CAR) of fluorotebuquine when compared with tebuquine within the parasite. Measurement of the cellular accumulation of the tebuquine analogues and seven related 4-aminoquinolines shows a significant relationship (r = 0.98) between the CAR of 4-aminoquinoline drugs and the reciprocal of drugs IC50.

The effect of fluorine substitution on the metabolism and antimalarial activity of amodiaquine.

Amodiaquine (AQ) (2) is a 4-aminoquinoline antimalarial which causes adverse side effects such as agranulocytosis and liver damage. The observed drug toxicity is believed to be related to the formation of an electrophilic metabolite, amodiaquine quinone imine (AQQI), which can bind to cellular macro-molecules and initiate hypersensitivity reactions. 5'-Fluoroamodiaquine (5'-FAQ, 3), 5',6'-difluoroamodiaquine (5',6'-DIFAQ,4), 2',6'-difluoroamodiaquine (2',6'-DIFAQ,5), 2',5',6'-trifluoroamodiaquine (2',5',6'-TRIFAQ, 6) and 4'-dehydroxy-4'-fluoroamodiaquine (4'-deOH-4'-FAQ,7) have been synthesized to assess the effect of fluorine substitution on the oxidation potential, metabolism, and in vitro antimalarial activity of amodiaquine. The oxidation potentials were measured by cyclic voltammetry, and it was observed that substitution at the 2',6'- and the 4'-positions (2',6'-DIFAQ and 4'-deOH-4'-FAQ) produced analogues with significantly higher oxidation potentials than the parent drug. Fluorine substitution at the 2',6'-positions and the 4'-position also produced analogues that were more resistant to bioactivation. Thus 2',6'-DIFAQ and 4'-deOH-4'-FAQ produced thioether conjugates corresponding to 2.17% (SD: +/- 0.27%) and 0% of the dose compared with 11.87% (SD: +/- 1.31%) of the dose for amodiaquine. In general the fluorinated analogues had similar in vitro antimalarial activity to amodiaquine against the chloroquine resistant K1 strain of Plasmodium falciparum and the chloroquine sensitive T9-96 strain of P. falciparum with the notable exception of 2',5',6'-TRIFAQ (6). The data presented indicate that fluorine substitution at the 2',6'-positions and replacement of the 4'-hydroxyl of amodiaquine with fluorine produces analogues (5 and 7) that maintain antimalarial efficacy in vitro and are more resistant to oxidation and hence less likely to form toxic quinone imine metabolites in vivo.

Novel, potent, semisynthetic antimalarial carba analogues of the first-generation 1,2,4-trioxane artemether.

Ten novel, second-generation, fluorinated ether and ester analogues of the potent first-generation analogues artemether (4a) and arteether (4b) have been designed and synthesized. All of the compounds demonstrate high antimalarial potency in vitro against the chloroquine-sensitive HB3 and -resistant K1 strains of Plasmodium falciparum. The most potent derivative 8 was 15 times more potent than artemisinin (2) against the HB3 strain of P. falciparum. In vivo, versus Plasmodium berghei in the mouse, selected derivatives were generally less potent than dihydroartemisinin with ED(50) values of between 5 and 8 mg/kg. On the basis of the products obtained from the in vitro biomimetic Fe(II)-mediated decomposition of 8, the radical mediator of biological activity of this series may be different from that of the parent drug, artemisinin (2).

Mechanism-based design of parasite-targeted artemisinin derivatives: synthesis and antimalarial activity of benzylamino and alkylamino ether analogues of artemisinin.

Several artemisinin derivatives linked to benzylamino and alkylamino groups were synthesized in order to enhance accumulation within the malaria parasite. The in vitro antimalarial activity was assessed against the chloroquine sensitive HB3 strain and the chloroquine resistant K1 strain of Plasmodium falciparum. In general the incorporation of amino functionality enhances the activity relative to artemisinin. The most potent analogue in the series was compound 6 which was severalfold more active than artemisinin against both strains of P. falciparum used in the study.

The effect of fluorine substitution on the hepatotoxicity and metabolism of paracetamol in the mouse.

The widely used analgesic paracetamol (P) produces fulminant hepatocellular necrosis in humans when taken in overdose. The toxicity is mediated by drug oxidation and depletion of hepatic glutathione. We have, therefore, explored the effects of fluorine substitution on the hepatotoxicity of P in female CD1 mice. 3-Fluoro-4-hydroxyacetanilide (1FPO), 3,5-difluoro-4-hydroxyacetanilide (2FPO), 2,6-difluoro-4-hydroxyacetanilide (2FPN) and 2,3,5,6-tetrafluoro-4-hydroxyacetanilide (4FP) were synthesized, characterized and investigated for their potential to cause hepatotoxicity in the mouse. Introduction of fluorine into P increases the oxidation potential of the drug. The oxidation potentials of paracetamol and its fluorinated analogues were measured by cyclic voltametry and found to increase in the order P < 1FPO < 2FPO < 2FPN < 4FP. Serum transaminase (ALT) and hepatic glutathione were measured 24 and 6 hr, respectively, after administration of a single dose (2.65 mmol/kg) of each compound to female CD1 mice. There was significant elevation of ALT in mice given P, 1FPO and 2FPO, but not in those which received either 2FPN or 4FP. Hepatic glutathione was reduced significantly by administration of P and IFP, but not after administration of 2FPO, 2FPN or 4FP. Accordingly, glucuronide and sulphate conjugates, but not thioether metabolites, were detected in urine after administration of 14C-labelled 2FPO, 2FPN and 4FP. These data indicate that introduction of fluorine into the 2 and 6 positions increases the oxidation potential of paracetamol which in turn reduces the propensity of the molecule to undergo oxidative bioactivation, and thereby reduces the in vivo toxicity of the molecule.

The effect of fluorine substitution on the physicochemical properties and the analgesic activity of paracetamol.

The physicochemical properties and analgesic action of six fluorinated analogues of 4-hydroxyacetanilide (paracetamol) have been investigated. Fluorine substitution adjacent to the hydroxyl group increased lipophilicity and oxidation potential whilst substitution adjacent to the amide had little effect on lipophilicity but led to a greater increase in oxidation potential. Lack of coplanarity and conjugation of the amide group and aromatic ring was also apparent with the analogues that had fluorine in the 2 and 6 positions. Introduction of fluorine into the amide group of paracetamol increased the lipophilicity 4-fold and also increased the oxidation potential of paracetamol. ED50 values for analgesic activity in the phenylquinone-induced abdominal constriction test on male Swiss White mice showed that ring substitution by fluorine reduced activity, especially at the 2,6-positions. Introduction of fluorine into the amide group enhanced activity significantly. Correlation of the analgesic activity with the physicochemical properties indicated that conjugation (and planarity) of the amide group with the aromatic ring is essential for activity and that ease of oxidation may also be an important factor.

An investigation into the haematological toxicity of structural analogues of dapsone in-vivo and in-vitro.

With microsomes prepared from a single human liver, 4,4'-diaminodiphenyl sulphone (DDS), 4-acetyl-4-aminodiphenyl sulphone (MADDS), 4-acetyl-4-aminodiphenyl thioether (MADDT) and 4,4'-diacetyldiphenyl thioether (DADDT) caused significantly greater methaemoglobin formation compared with control. In-vitro in the rat, the pattern of toxicity was slightly different:DADDT was not haemotoxic, whilst 3,4'-diaminodiphenyl sulphone (3,4'DDS) and 3,3'-diaminodiphenyl sulphone (3,3'DDS) as well as DDS, MADDS and MADDT were significantly greater than control. 4,4' Acetyl diphenyl sulphone (DADDS), 4,4' diaminodiphenyl thioether (DDT), 4,4'-diaminodiphenyl ether (DDE) and 4,4' diaminooctofluorodiphenyl sulphone (F8DDS) did not cause significant methaemoglobinaemia in either human or rat liver microsomes. DDS, MADDS, and MADDT were not significantly different in haemotoxicity generation in-vitro in the presence of human microsomes. In the rat in-vitro, DDS, MADDS, and 3,4'DDS did not differ significantly in red cell toxicity, and were the most potent methaemoglobin formers. The 3,3'DDS and MADDT derivatives were both significantly less toxic compared with DDS. None of the compounds tested caused haemoglobin oxidation in the absence of NADPH in-vitro. In the whole rat, DDS, MADDS and MADDT caused significantly higher levels of methaemoglobin compared with control. None of the remaining compounds caused methaemoglobin formation which was significantly greater than control. DDS and MADDS were the most potent methaemoglobin formers tested, in-vivo and in-vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of chemical substitution on the metabolic activation, metabolic detoxication, and pharmacological activity of amodiaquine in the mouse.

The adverse reactions associated with the antimalarial amodiaquine (AQ), agranulocytosis and hepatotoxicity, have been attributed to the bioactivation of the drug to a quinone imine metabolite. Therefore the effect of chemical modification on the metabolism of AQ was studied, with particular reference to the prevention of bioactivation and the introduction of glucuronidation. Glutathione conjugates of AQ and desethylAQ were eliminated in bile after intraportal administration of [3H]AQ (54 mumol/kg, 20 microCi/kg) to anesthetized male CD1 mice. Thioether conjugates excreted into bile over 3 h accounted for 28% of the administered dose. Fluorine substitution at the C-4 position of AQ blocked bioactivation, as measured by formation of thioether conjugates, and resulted in a 5-fold decrease in biliary excretion of radiolabeled dose: ca 6% versus ca 29%. Additional substitution of a primary alcohol function into one of the ethyl moieties introduced glucuronidation as a pathway of elimination, with 10% of the dose being excreted in bile as an O-glucuronide of the parent compound over a 3-h period; excretion of total radioactivity in bile increased 2.5-fold. These substitutions resulted in a 2-fold greater excretion of radiolabel into urine: 41% and 39% for DFAQ and HDFAQ, respectively, versus 23% for AQ. Novel carboxylic acid and N-oxide metabolites of the fluorinated analogues were identified. AQ and the two fluorinated analogues had similar activity against Plasmodium berghei in mice. These results demonstrate that the metabolism of AQ can be diverted from extensive bioactivation to direct detoxication by simple chemical substitutions that do not impair pharmacological activity.