Comparison of lifibrol to other lipid-regulating agents in experimental animals

345

COMPARISON OF LIFIBROL TO OTHER REGULATING AGENTS IN EXPERIMENTAL

LIPIDANIMALS

BRIAN R. KRAUSE,* RICHARD BOUSLEY, KAREN KIEFT, DONALD ROBERTSON,? RICHARD STANFIELD, ELLEN URDAT and ROGER S. NEWTON

of Atherosclerosis Therapeutics and ‘cPathology and Experimental To.\-ic,olocg_v,Purke-Duvis Pharmaceuticul Reseurch, 2800 Plymouth Road, Ann Arbor, MI 48105, USA

Dcpartmcnts

SUMMARY In vitro data suggests that lifibrol lowers plasma cholesterol by inhibiting cholesterol synthesis. We report that lifibrol is far less potent in vitro and in viva than lovastatin for inhibiting ‘“C-acetate incorporation into sterols. Moreover, several major differences between lifobrol and lovastatin were noted in various animal models. In contrast, lifibrol exhibited several activities in common with gemfibrozil, another phenoxy-acid-type drug. Specifically, in normal rats lifibrol, like gemfibrozil, lowered plasma non-HDL-cholesterol and triglycerides, and increased liver weight and hepatic peroxisomal marker enzyme activities. Lovastatin only lowered plasma triglycerides. In cholesterol-fed rats lifibrol and gemfibrozil lowered non-HDL-cholesterol and elevated HDL-cholesterol while lovastatin was inactive. Finally, lovastatin but not lifibrol exhibited hypocholesterolemic activity in normal guinea pigs and resin-primed dogs. Our interpretation is that these data do not support the notion that lifibrol lowers plasma cholesterol in \jivo by inhibiting cholesterol synthesis. KEYWORDS: cholesterol, HMG-CoA reductase, fibrates, hypolipidernic, triglycerides, rat.

INTRODUCTION Lifibrol (K12.148), an analogue of terbufibrol [ 11, has been shown to lower low density lipoprotein-cholesterol (LDL-C) by up to 35% in healthy human volunteers without affecting high density lipoprotein-cholesterol (HDL-C) or plasma triglycerides [2]. Based upon in vitro experiments, the mechanism by which lifibrol lowers plasma cholesterol, as with the analogues [ 1, 31, is thought to be inhibition of cholesterol synthesis [4]. However, structurally lifibrol is a phenoxy acid and therefore bears some resemblance to the fibrate class of lipid“For correspondence 1043-66 18/94/040345-l

3/$0X.00/0

0 1994 The Italian Pharmacological

Society

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346

(CH3)3C

CH2CH27HCH2

O-(=&,0,

OH LIFIBROL

7H3

0CH2CH2CH2CC02H A H3

CH3

LOVASTATIN

GEMFIBROZIL

Fig. 1.

Chemical structures of lifibrol, lovastatin and gemfibrozil.

lowering drugs. In the present study we compared the pharmacologic activities of lifibrol, lovastatin and gemfibrozil (Fig. 1) in various animal models to determine if the overall profile would be the one expected of a cholesterol synthesis inhibitor (e.g. lovastatin) or a fibric acid derivative (e.g. gemfibrozil).

MATERIALS

AND METHODS

Sterol-synthesis

The techniques used to evaluate the effects of the drugs on sterol synthesis in and in viva have been described previously [5]. Briefly, the ability of compounds to inhibit cholesterol synthesis in vitro was determined using partially purified liver homogenates from cholestyramine-treated rats. The incorporation of “C-acetate into non-saponifiable lipids (NSL) in the presence of required cofactors was determined in the absence (controls) or presence of the drugs as a measure of cholesterol synthesis. For the in vivo assessment of sterol synthesis ‘“C-labelled acetate was injected intraperitoneally 1 h after a single oral dose of the drug in an aqueous, carboxymethylcellulose (CMC) (1.5% w/v) vehicle. The radioactivity in plasma NSL was then determined 50 min after isotope injection in vehicle controls and drug-treated animals. Percentage inhibition was calculated at each of four doses. I&,, values (in vitro) and EDso values (in vi\w) were calculated from these data as previously described [6]. \litr.o

In \yilw evaluation of the drugs included assessment in normal, chow-fed rats 17, S] and guinea pigs 191, cholesterol-fed rats [lo], and cholestyramine-primed dogs [ 1 I, 121. Chow-fed rats (male Sprague-Dawley, 200-225 g) and guinea pigs (Hartley strain, 450-500 g) were dosed by daily gavage (15:00-16:OO hrs) for 2 weeks using an aqueous CMC (1.5% w/v) vehicle. Cholesterol-fed rats received chow supplemented with cholesterol (l.S%), cholic acid (O.S%, and peanut oil (5.5%) for 2 weeks with daily gavage (CMC vehicle) dosing during the second

Pharmucolo~ical

Research.

Vol. 29, No. 4, 1994

347

week (i.e. drug administration in rats with pre-established dyslipidemia). The dogs (female beagles, 4-6 kg) were meal-fed (08:00-12:00 hrs) a normal diet containing 12 g of cholestyramine for 3-4 weeks until plasma cholesterol levels plateaued. Then the animals were allocated to groups with approximately the same plasma cholesterol level (Allocate software, Elsevier), and drugs were administered daily by capsules 4 h after the end of the each daily meal for 1 week. Peroxisome-associated enzymes In a separate experiment using chow-fed rats the drugs were administered at 100 mg kg-’ day-’ for 1 week and then the livers were removed to prepare sucrose homogenates as described by DeAngelo et al. [ 131. Cyanide-insensitive P-oxidation was measured according to the procedure of Lazarow [ 141, and carnitine acetyltransferase was assayed according to Bieber and Fiol [15]. Lipidllipoprotein analyses In all rodent experiments (Tables II, III and V) blood was obtained in the nonfasted state 24 h after the last drug dose for determining plasma cholesterol [16] and triglycerides [17]. Cholesterol distribution was determined either by dextran precipitation (HDL-C, non-HDL-C) [ 181 or by high-performance gel chromatography (VLDL-C, LDL-C, HDL-C) using a Superose 6HR column and an on-line cholesterol reagent [ 191.

RESULTS As expected, lovastatin potently inhibited sterol synthesis in liver homogenates. The I& was about 20 nM (Table I). Lifibrol exhibited weak in vitro inhibitory activity, while gemfibrozil was inactive. When sterol synthesis was measured in vivo after a single drug dose lovastatin was again the most potent, and lifibrol and gemfibrozil were weaker and approximately equipotent.

Effect of lifibrol, COWlpOU?ld

Lifibrol Lovastatin Gemfibrozil

lovastatin,

Table I and gemfibrozil on sterol synthesis in vivo in the rat

in vitro and

In vitro* (ICsr,, PM)

In vivot (EDS,,, m‘q W)

16.20 0.02 >lOO

14.4 0.9 19.7

Values are the calculated IC5,,s or ED5,,s derived from six concentrations (in l’ifw) and three doses (in l~iw). *“C-acetate incorporation into non-saponifiable lipids (NSL) using partially purified liver microsomes. iIncorporation of IP administered ‘“C-acetate into NSL I h after a single, oral dose of drug.

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348

In rats fed a normal diet, gemfibrozil and lifibrol, but not lovastatin, lowered plasma non-HDL-C concentrations (Table II). Gemfibrozil elevated HDL-C at two out of three doses, and this resulted in a slight increase in total cholesterol at the highest dose. This increase in HDL-C was confirmed using HPGC; by this method VLDL-C, LDL-C, and HDL-C accounted for 1 l%, 17% and 72% of total cholesterol in controls, but 3%, 8% and 89%, respectively, for gemfibrozil-treated rats (100 mg kg-‘). The absolute HDL-C value in control animals was 42 mg dll’ and this value increased to 59 mg dll’ with the highest dose of gemfibrozil (data not shown). With lifibrol plasma total cholesterol decreased by 45% at 100 mg kg-’ since no elevation of HDL-C occurred. Lifibrol was more efficacious than gemfibrozil for lowering non-HDL-C (i.e. significant differences between drugs at 100 mg kg-‘). Plasma triglyceride concentrations decreased dosedependently with all three drugs; gemfibrozil was more potent than lovastatin but equipotent to lifibrol. Liver weights in these normal rats increased by 1527% with gemfibrozil and by 1536% with lifibrol (the difference between drugs was not significant), but did not change with lovastatin treatment (data not shown). We have previously reported that gemfibrozil has no plasma-lipid-regulating activity in normal, chow-fed guinea pigs [12], an animal in which almost all plasma cholesterol is transported in LDL [9]. The data in Table III show that this is also the case for lifibrol. Lovastatin, on the other hand, lowered plasma cholesterol by up to 36% (30 mg kg-‘). Similar results were found in resin-primed dogs, which are very sensitive to HMG-CoA reductase inhibitors [ 11, 121. Lifibrol was inactive at doses lOO-fold higher than those required for lovastatin (Table IV). Although not evaluated in resin-primed dogs, gemfibrozil, unlike lovastatin [ 111,

Table II activities of lifibrol, lovastatin and gemfibrozil in normal, chow-fed rats

Lipid-regulating Treutment group

Dose (mg !I$) _

Plasma c,holesterol (mg dl-‘)

Non-HDL-C (mg dl-‘)

HDL-C (mg dl-‘)

Plasma triglycerides (mg dl ‘)

Controls

-

64f3”

25k2”

40+2”.”

207f8”

Gemfibrozil Gemfibrozil Gemfibrozil

10 30 100

6 1k2d.h 51+3h.’ 68f7”

12+1’.d 9+2’,” 14+4’

48f 1c.d 42f2”,’ 54+5*

127+10b.’ 99f 13c.d.c 96f 13d.’

Lovastatin Lovastatin Lovastatin

10 30

I 00

62f6’,b 71f6” 63+5”,b

25+3” 33+4b 29f3”,b

37+3”,h 4 I ,2”.b 34+3b

190+13” 151f13h 127f13b.C

Lifibrol Lifibrol Lifibrol

IO 30 100

65f4” 42+4’,* 36f4d

26f2”.h 6+2*,” lf2’

39rt-2“,’ 36f4”.h 35+2”,h

122f5b,‘,d 92+gd,’ 74+6”

Values (mean?sEM, n=h per group) sharing a common superscript are not significantly different (P
Pha~mcr~olo,qi~~il Reserrrch. Vol. 29, No. 4. I994

349

does not lower plasma cholesterol in chow-fed dogs at 25 mg kg-’ (data not shown). Both lifibrol and gemfibrozil failed to significantly alter cholesterol concentrations or distribution in cholesterol-fed rats at doses of 1 and 3 mg kg-‘, but both drugs were active at 10 and 30 mg kg-’ (Table V). Lifibrol was more efficacious than gemfibrozil for both lowering non-HDL-C and elevating HDL-C. In a second experiment (experiment 2) lovastatin was inactive in this animal model but lifibrol was again active. Changes in cholesterol distribution were

Effects

of lifibrol

Table III and lovastatin on plasma cholesterol normal, chow-fed guinea pigs

Treutment ~,‘OU/J Eq~eriment

Dose (mg k-‘)

Cholesterol (mg dl-‘)

Triglycerides (mg dl-‘)

10 30

47+4 73f16 55f5

IlOfll 114f13 139fl8

3 IO 30

44f3 38f3 32f3* 28+4*

154f12 173f14 159+28 140f13

in

1

Controls Lifibrol Lifibrol Experiment Controls Lovastatin Lovastatin Lovastatin

and triglycerides

2

Values are the meanfsr+r, n=7-8 animals per treatment *Significantly different from controls (P
group.

Table IV Effect of lifibrol and lovastatin on plasma cholesterol cholestyramine-primed beagle dogs Treatmerlt group

Dose (nq kg-‘)

Plasma cholesterol (mg dl-‘) Day 0

E.vpcriment Controls Lifibrol Lifibrol

I

Experiment Controls Lovastatin Lovastatin Lovastatin

2

10 30

0.1 0.3 1.0

in

Day 7

Percentage

112*11 103fl I 109f14

125fl5 107+12 102f14

+I2 +4 -6

1OSf 10 I OOf8

103k12 x7+9 88+10* 76fl2*

-2 -13 -21 -33

112fll 114+11

Values are the meankst+t. N=.%6 per group. *Significantly different from pretreatment (P
(day 0) value using paired,

c,hongr

two-tailed

t-test

P /1NI.t7I(I(.O/O,si(.(Il Kc.rcwr-t il. \;I/. 20. NC,.4. /c)YJ

350

Effect of lifibrol,

lovastatin

Table V and gemfibrozil

TPC

(IW) (nq dl-‘)

_~ E.\-lwrinwnr

HDL-C W) (q ___-

tllr’)

L’LDL-C (HPGC) (my dl

rats

’)

HDL-C (HPGC) (n7g dl ‘)

1

CF controls Chow controls

-

I

Lifibrol Lifibrol Lifibrol Lifibrol

30

GemfibroLil Gemfibrozil Gemfibrozil Gemfibrozil

3 10 30

Exper-itnrnt

Non-IfDL-C

in cholesterol-fed

3

10

1

2 17f25” 69f3”

200&26” 29+2’

182+14”~” 1YOf I P I29tY’ 121*11’

I 62+ 13’1.” 167,I3” ” 82?9” 6SW

2O?l” 23kl” 47&S” 57?5’

16Sk 13”,“ 174*14”h 74+ I p 48@’

lYf2” 73f]J ._&_ S3fXh 7lf8’

lxXfl.~“~h 186f28”.h I 7 1f I 2“.h. I 49f I 3 h.L

16Y+16”,h I 67?29” ” I 46k2Xh,’ 101*13’~“

lYf2” 19k2” 2Sk.3” 38+_?

170+1s”,h I76-t3 I .‘.h I 53f27h.’ I oo* 1.3‘.”

19k2” 20+2” 25+-+” sof4h

x0+24” 29fl’

2

CF controls Chow controls Lovastatin Lovastatin Lifibrol

30 100 30

269+37’ 65% 264f2Y” 242?S” I 33+7h

I3k2” 45+4”

13?1” 1 l?l” 641t7’

Values arc the mean*sEbq (11=6 per group). For each column in a given experiment. numbers sharinp ;I common superscript are not significantly different (P
in experiment I using two independent methods. Unlike previous [IO], HPCG analysis revealed one major VLDL peak. and a separate LDL peak or shoulder was not detectable. Averaged HPGC profiles (,1=6 per group) are shown in Fig. 2 I’or both control groups and the two drugs at IO mg kg ’ (experiment 1. Table V). At this dose it i\ clear that a greater percentage of cholesterol was found in HDL-C for lifibrol (42%) compared to gemfibrozil (16%). At the highest dose (30 mg kg ‘) the IIPGC profile for lifibrol (not shown) resembled that found in animals hwitched to ~1normal chow diet during the second week (59% HDL-C. 41(/c non-HDL-C). and the absolute values for HDL-C were significantly higher in the lifibrol 5(Troup (71 mg dl ‘) compared to the thou controls (41 mg dl !) (Table V). In addition to prol‘iling these drug\ for lipid-regulating activity. we also examined change” in peroxisomal marker enzymes. Anion g representative fibric acid derivative<. lifibrol increased the activity of carnitine acetyltransferase to the same extent as gemfibroril. clofibrate. bczafibrate and fenofibrate in chow-fed rats detertnined

experiments

(Fig. 3). Hepatic cyanide-insensitive P-oxidation was increased only about threefold by lifibrol, clofibrate and gemfibrozil, whereas bezafibrate and fenofibrate increased activity l&fold. Ciprofibrate was by far the greatest inducer of both activities (> 100-fold). In this experiment all drugs decreased plasma triglycerides (by 51-690/o) and increased liver weights (by 23-36%), except clofibrate which was less active (-25% in triglycerides, 16% liver weight gain). Plasma total cholesterol concentrations decreased by 14, 19, 24, 34, 53 and 55% with gemfibrozil, clofibrate, bezafibrate, fenofibrate, ciprofibrate and lifibrol, respectively.

DISCUSSION The present experiments confirm that lifibrol acetate into sterols in ~itr.o at low micromolar

inhibits the incorporation of “Cconcentrations. We extended this

Fig. 2. Cholc~terol dixtribution as detcrmincd by HPGC for (a) cholesterol-fed, (b) chow-fed, and rat\ fed cholesterol and treated with (c) lifibrol and (d) gemfibro/il (average curves for six ;m~mal\ per group). The mean plasma total cholesterol (TC) values are (a) 218, (b) 70, (c) 128, (tl) 17X mg dl ’and the percentapes of total cholesterol found in VLDL (17-21 min) and HDL (30-il min) are indicated. Note that the absorbance (500 nm) is a direct measure of cholesterol in lipoprotein particles using the on-line cholesterol reagent.

352

Pha~niuc.ologic~al

45(a) 40 35 30; /, /

Vol.

29, No.

4, 1994

d

d

c b

z -

Reseat-ch.

lo-

CON

GEM CL0

LIF

BE2

FEN

CIP

0

(‘0%

GEM CL0

LIF

BEZ

FEN

(‘IP

Fig. 3. Liver (a) camitine acetyltransferase and (b) cyanide-insensitive-P-oxidation activities in normal tats treated (100 mg kg-‘, CMC vehicle, I week) with gemfibrozil (GEM), clofibrate (CLO), lifibrol (LIF), bezafibrate (BEZ), fenofibrate (FEN) and ciprofibrate (UP). Bars with different superscripts are significantly different using a Student-Newman-Keuls post-hoc multiple comparison (P4.05). Values are the meansfsEM (n=5 per group). CON=controls.

observation by also determining the in rjivo incorporation of the same radiolabel. This inhibition is thought to be at a step prior to the formation of HMG-CoA in the cholesterol synthetic pathway [4, 201. The novel finding, however, is that this degree of inhibition of sterol synthesis by lifibrol does not translate into the expected efficacy in animal models in which the potent inhibitor of cholesterol synthesis, lovastatin, is active. In fact, the overall in \k) profile of lifibrol resembles gemfibrozil more than it does lovastatin. As a first example, lifibrol and gemfibrozil both lower plasma non-HDL-C and triglycerides in normal rats whereas lovastatin lowers only triglycerides. The in rlivo sterol synthesis data argues against the notion that lovastatin is poorly absorbed in rats on a normal diet and that this would account for its lack of hypocholesterolemic activity. In addition, lovastatin has been shown to lower plasma triglycerides in normal rats by decreasing hepatic VLDL-triglyceride secretion [21]. Finally, both compounds were administered to normal rats in the same experiment just prior to the dark cycle when sterol synthesis is maximal. Therefore, it is likely that differences between lifibrol and lovastatin in this model are due to inherent differences in mechanism rather than to differences in drug absorption, hepatic drug exposure or time of dosing [4]. The apparent similarity between lifibrol and gemfibrozil in normal rats with respect to lipid lowering may indicate the existence of at least one common mechanism. However, in the present study, gemfibrozil but not lifibrol, elevated plasma HDL-C from 20 to 40% in normal rats at the high dose. In this regard, lifibrol is more similar to other drugs of the fibrate class. Previously we reported an increase of only 6% at 125 mg kg-’ with gemfibrozil, while HDL-C was reduced by ~20% by clofibrate, bezafibrate and fenofibrate [S]. The previous experiment was of l-week duration, in contrast to the present 2-week study. Others have recently observed an increase in plasma total cholesterol, presumably

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353

HDL-C, as well as apoE concentrations in normal rats administered high doses of gemfibrozil but not clofibrate or fenofibrate [22]. We have recently observed similar differences between gemfibrozil and bezafibrate (Krause and Bisgaier, unpublished observation). Thus, pharmacologic effects on HDL metabolism in normal rats may differ between lifibrol and gemfibrozil, as it does between gemfibrozil and other fibric acid derivatives. Similarly, it is clear from Table 1 that gemfibrozil, unlike lifibrol, has no direct inhibitory effect on sterol synthesis in ~~itm but yet inhibited sterol synthesis acutely in viva to a similar extent as lifibrol. Thus, although we have not measured sterol synthesis after chronic dosing, and compensatory mechanisms may exist, it appears that the mechanisms for inhibition of sterol synthesis in viva, like HDL-C elevation, are also likely to be different for gemfibrozil and lifibrol. The results for gemfibrozil are reminiscent of those reported by Stange et al. [23] who found that HMG-CoA reductase activity was inhibited in mononuclear cells from patients treated with the drug, but yet direct addition of the drug to cells in vitro had no effect on enzyme activity. If inhibition of cholesterol synthesis represented the primary mechanism for the lipid-regulating activity of lifibrol [4], one would also not expect activity in cholesterol-fed rats since the more potent synthesis inhibitor, lovastatin, was inactive in this model. To our knowledge this is the first time that the inactivity of lovastatin in cholesterol-fed rats has been reported. In contrast, gemfibrozil, and especially lifibrol, are active in this animal model, and both drugs also elevated the diet-induced low levels of HDL-C. Therefore, the responses to gemfibrozil and lifibrol were qualitatively similar in cholesterol-fed rats, but lifibrol was more efficacious. It remains to be determined if this is due to differences in drug absorption or rate of drug metabolism in this animal model. The primary mechanism for the HDL-C elevating activity of gemfibrozil is unknown, but unlike other fibrates, it has been associated with changes in the concentration and distribution of apoE [22,24, 251. With regard to non-HDL-C lowering, gemfibrozil has been shown to decrease hepatic apoB gene expression irz vitro in HepG2 cells 1261 and in viw in chow-fed rats [22]. It remains to be determined whether lifibrol shares any of these potential mechanisms. Lovastatin has been shown to decrease plasma cholesterol and to up-regulate hepatic LDL receptors, resulting in enhanced clearance of LDL, in normal, chowfed guinea pigs [9]. Again, since this is due to inhibition of cholesterol synthesis, one might expect lifibrol to be active in this animal model. However, lifibrol was inactive. The same situation pertains to the cholestyramine-primed dog in which enhanced LDL clearance has been demonstrated for lovastatin 1271. It seems unlikely to us that the lack of activity of lifibrol in guinea pigs and dogs is due to poor drug absorption since the closely related analogue, terbufibrol, possesses good bioavailability in dogs and the oral LDso in guinea pigs (320 mg kg-‘) is the lowest of all species studied [28]. With regard to mechanism, it seems reasonable to assume that inhibition of cholesterol synthesis in guinea pigs and dogs by lovastatin is associated with increased mRNA for the LDL receptor as occurs in hamsters and rabbits on normal diets 1291. Presumably liver cells are depleted of a regulatory sterol that acts as a repressor of the LDL receptor gene, leading to increased production of LDL receptors. Since lifibrol is inactive in these LDL models, it is possible that this drug does not inhibit cholesterol synthesis potently

enough i/l I’~~IYJ(IC,,,=16 ,u~I) or in \,i\v to induce similar alterations in gene expression. Lifibrol might, however, directly up-regulate LDL receptors independent of changes in cholesterol synthesis 1301, and an animal mode1 in which receptors are down-regulated might be necessary to demonstrate in \~ivo efficacy (e.g. casein-fed rabbits, [12]). But, in the absence of such data we would predict. based upon our in \,ivo profiling, that, like gemfibrozil [22, 231, lifibrol would have no effect on LDL-receptor-mRNA or on the binding and degradation of LDL in \litt.o or in \*ivo. The last observation that supports the notion that lifibrol is related pharmacologically to gemfibrozil is that both drugs increase liver weights and pcroxisomal-related enzyme activities to the same extent in normal rats at high doses. Other have reported in preliminary form that compounds of the lifibrol class (e.g. terbufibrol) produce mild hepatomegaly [31], but yet lifibrol itself had no apparent effect on hepatic catalase activity 1321. Our data using more sensitive peroxisome markers leave little doubt that lifibrol is at least a mild enzyme inducer. Since compactin does not induce peroxisomal enzymes at even higher doses 1331 we did not determine enzyme activities for lovastatin, but there was no change in liver weights with lovastatin, and hepatomegaly is a common feature, if not a prerequisite, for hypolipidemic peroxisome proliferators [34]. In comparison to other fibric acid derivatives, the changes in enzyme activities due to gemfibrozil and lifibrol were similar to clofibrate, but quantitatively less than those for bezafibrate and fenofibrate. and far less than ciprofibrate. Although this hepatic response cannot be demonstrated in man for clofibrate [35], gemfibrozil (361 or fenofibrate 1371 and is therefore thought to be of questionable clinical relevance [3X. 391. it nonetheless allows classification of compounds preclinically, as done in the present report. Moreover. the chemical structure of lifibrol resembles that of gemfibrozil. and even more so that of bezafibrate which also contains two phenyl rings. Our data are therefore consistent with lifibrol being a member of the diverse class of agents known as peroxisome proliferators, but like gemfibrozil [40, 411 it is a weak member of this class. Although a good correlation has been described between cyanide-insensitive P-oxidation and peroxisome enlargement 1391. our biochemical data require ultrastructural data for confirmation. It also remains to be determined if either gemfibrozil or lifibrol show direct binding to the peroxisome proliferator activated receptor (PPAR) [42]. In summary, we have been unable to provide evidence consistent with the view 141 that lifibrol lowers plasma cholesterol by inhibiting cholesterol synthesis. In fact. qualitatively. both the plasma lipid-regulating and hcpatic peroxisomal rexpon\cs resemble those found with gemfibroLil rather than lovastatin, a known potent synthesis inhibitor [ I I J. One difference between gemfibroril and lifibrol. however, was that HDL-C elevation waa observed in normal rats for gcmfibrozil but not lifihrol. Thus. lifibrol resembles other fibrates with respect to HDL-C rcspon~e in normal rats. In humans. the HDL response also appears to be different since gemfibroTil (431. but not Iifibrol [I?], has been reported to elevate HDL>-C in normolipidl~niic human subjects. In contrast, if the data in cholesterol-fed rat> is predictive one might cxpcct greater changes in non-HDL-C and HDLC with IiUibrol compared to gemfibroLi1 in dyslipidemic subjects. It will be of interest L\hcthcr this prediction is borne out in futurt‘ clinical trials.

ACKNOWLEDGEMENTS The authors are grateful to Erika Ferguson and Mary Kay Shaw-Hes for determining the effects of gemfibrozil, lovastatin and lifibrol on sterol synthesis in \,jt~o and in ~~i\,o, respectively. We also thank Bruce Roth and Pat O’Brien (Department of Chemistry) for helpful discussions relating to the structural features of hypolipidemic drugs.

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