Novel nonnucleoside inhibitors of HIV-1 reverse transcriptase. 8. 8-Aryloxymethyl- and 8-arylthiomethyldipyridodiazepinones

Nevirapine (I) is the first human immunodeficiency virus type 1 (HIV-1) nonnucleoside reverse transcriptase (RT) inhibitor to reach regulatory approval. As a result of a second generation program around the tricyclic core system of nevirapine, 2-chloro-5, 11-dihydro-11-ethyl-5-methyl-8-(2-(pyridin-4-yl)ethyl)-6H-dipyrido[3, 2-b:2',3'-e][1,4]diazepin-6-one (II)1a and 2-chloro-5, 11-dihydro-11-ethyl-5-methyl-8-phenylethyl-6H-dipyrido[3,2-b:2', 3'-e][1,4]diazepin-6-one (III)1a were identified as broad spectrum HIV-1 RT inhibitors. A detailed examination of replacing either of the methylenes of the 8-ethyl linker of II or III is presented. It was found that 8-aryloxymethyl and 8-arylthiomethyl are the preferred pattern of substitution for potency against RT. The most potent compounds were further evaluated against a panel of clinically significant mutant RT enzymes (K103N, V106A, G190A, P236L) and in cytotoxicity and in vitro metabolism assays. The most potent compound was 2-chloro-8-phenylthiomethyl analogue 37 which displayed sub-100 nM activity against all HIV-1 RT enzymes tested.

Novel nonnucleoside inhibitors of HIV-1 reverse transcriptase. 7. 8-Arylethyldipyridodiazepinones as potent broad-spectrum inhibitors of wild-type and mutant enzymes

Like other nonnucleoside inhibitors of HIV-1 reverse transcriptase, the dipyridodiazepinone nevirapine (Viramune, 1) selects for drug resistant variants of HIV-1, both in cell culture and in patients. In particular, the mutation of residue 181 from tyrosine to cysteine (Y181C) is associated with resistance to most reported nonnucleoside inhibitors. Introduction of an arylethyl substituent at the 8-position of the tricyclic dipyridodiazepinone skeleton confers enhanced potency against Y181C RT. Several analogues of this series display good broad spectrum potency against a panel of mutant enzymes.

[Erythropoietin and obstetric factors--evaluation with special reference to sex differences]

OBJECTIVE

We evaluated fetal Erythropoietin (EPO) in a clinical obstetrical environment, aiming to collect more information upon its role in diagnosis of fetal stress. As it is known that EPO cannot pass the placental barrier, we restricted to measure the concentration in the umbilical artery (UA) immediately after delivery.

PATIENTS AND METHODS

EPO was correlated to relevant obstetrical parameters. We looked for each parameter that was tested for differences in fetal sex. For this purpose, the statistics were performed by means of the Behrens-Fisher-Test (meaning a special application of the Student-T-Test). This device is an algorithm developed to compare regressions. For significance, a confidence level of less 5% was accepted. For the rest, we only used descriptive devices.

RESULTS

We found weak but positive correlation to gestational age for both sexes. No correlation was calculated for the duration of parturition or the birth weight. Highly significant differences were found for respiratory values of blood gases, taken from the UA when differentiated for fetal sex: males showed a strong and negative correlation for pO2 (r(m)= -0.45); the female EPO-level was not influenced by decreasing O2-level (r(w)= +0.06, p > 0.01). Similar results could be found for pCO2. (r(m)= +0.36; r(w)= -0.07; p < 0.01. Not significant, but comparable differences for the EPO-level in fetal blood were found for pH and base deficit.

CONCLUSIONS

If we accept that the EPO-level in fetal blood rises with relevant oxygen deficiency, then females seems to be better protected against damage from distress. This observation is congruent with the clinical experience from the authors who have the subjective impression that female sex is advantageous in harmful deliveries.

Recurrence of hepatitis B in liver transplants treated with antiviral therapy

BACKGROUND AND AIMS

In patients with terminal Hepatitis B Virus-related liver diseases, liver transplantation carries a consistent risk of Hepatitis B Virus recrudescence in the graft. In the attempt to reduce the reinfection rate with antiviral therapy, we studied a total of 16 viraemic patients.

PATIENTS AND METHODS

Twelve patients received Ganciclovir, starting 4-67 days (mean 25 days) before transplantation and prolonged for 10 days after transplantation; four patients were treated with Lactosaminated Arabinoside-Monophosphate 6 hours before surgery and prolonged for 28 days after surgery. All received hepatitis B immunoglobulins.

RESULTS

At transplantation, HBV-DNA had decreased to about 10(4) virus/ml (as assessed by the polymerase chain reaction assay) in 10 of the 12 patients treated with Ganciclovir. Of these patients, 4 died perioperatively from causes unrelated to Hepatitis B Virus reinfection. Of the eight survivors, only the patient who maintained a titre of 10(6) virus/ml at the time of transplantation developed viral recurrence 4 months after surgery. Before transplantation, 2 of the patients treated with Lactosaminated Arabinoside-Monophosphate had a viraemic load of 10(6) and 2 of 10(4) virus/ml. In all cases, viraemia became undetectable at the end of therapy. None died and Hepatitis B Virus recurred 2 months after transplantation in one. The overall rate of Hepatitis B Virus recurrence was 16.6%. The recurrence rate decreased to 9% in patients in whom the viraemic load decreased to around 10(4) virus/ml following treatment, compared to an overall recurrence rate of 50% in our historical series of patients transplanted for Hepatitis B Virus-related cirrhosis.

CONCLUSION

Antiviral therapy was effective in decreasing the risk of Hepatitis B Virus reinfection of the liver graft by decreasing the viral load before surgery.

Novel non-nucleoside inhibitors of human immunodeficiency virus type 1 reverse transcriptase. 6. 2-Indol-3-yl- and 2-azaindol-3-yl-dipyridodiazepinones

Modification of the non-nucleoside inhibitor of HIV-1 reverse transcriptase nevirapine (Viramune) by incorporation of a 2-indolyl substituent confers activity against several mutant forms of the enzyme.

Plasmid profile analysis and antibiotic resistance of Salmonella strains from clinical isolates in Cluj-Napoca

Resistance patterns, plasmid profiles and the genetic resistance determinants were investigated in 38 isolates of Salmonella enterica serotype Typhimurium and 19 isolates of Salmonella enterica serovar Enteritidis derived from children hospitalized in two clinics in Cluj-Napoca, during the period of 1995-1997. Incidence of plasmid and antibiotic resistance was very high in Salmonella typhimurium isolates. All strains were resistant to almost all antibiotics tested but susceptible to the third generation cephalosporines and fluoroquinolones. We identified three resistance patterns and six plasmid profiles. Each plasmid profile was characterized by the presence of two large plasmids of 150-180 Kbp. Approximately 60% of strains harbored three or four small plasmids of 1.3 to 9.5 Kbp. The plasmids of 8.5 Kbp encoded resistance to beta-lactam antibiotics and were non-conjugative. The other small plasmids were cryptic and also non-conjugative. Salmonella enteritidis isolates were susceptible to many antibiotics, except Tetracycline and Trimethoprim-Sulfamethoxazole. We identified three different resistance patterns but nine plasmid profiles. All plasmid profiles were characterized by the presence of a large plasmid (> 100 Kbp). The number and the diversity of small plasmids were higher than in S. typhimurium strains. There was no parallelism between resistance and plasmid profile: for the same resistance pattern a number of two or three plasmid profiles were found. Our conclusions are that Salmonella typhimurium strains were multiresistant to antibiotics and that many genetically different strains of Salmonella typhimurium and Salmonella enteritidis were responsible for gastroenteritis in children from Cluj County. The increasing antibiotic resistance highlights the need for more refined methods in genetic and epidemiological characterization of bacteria involved in gastrointestinal infections.

An unusual fibrillarin gene and protein: structure and functional implications

The diploid germinal nucleus of the ciliated protozoan Tetrahymena thermophila is unusual among eukaryotes in that it encodes a single copy of the gene for rRNA allowing identification of cis-acting mutations in rDNA affecting rRNA structure, function, and processing. The generally conserved nucleolar protein fibrillarin has been characterized from a number of systems and is involved in pre-rRNA processing. We have demonstrated that Tetrahymena has fibrillarin and have analyzed the cDNA and the genomic DNA encoding this protein. The derived amino acid sequence of the N-terminal region of Tetrahymena fibrillarin shows little similarity with the generally highly conserved glycine/arginine-rich N-terminal domain of other eukaryotic fibrillarins. The remainder of the amino acid sequence of the molecule is more conserved. Polyclonal antibodies generated against the full-length Tetrahymena fibrillarin expressed in bacteria recognize a protein of M(r) approximately 32,000 in whole-cell or nucleolar preparations. Immunocytochemistry localizes fibrillarin to nucleoli in the somatic macronuclei of vegetative cells. Transformation experiments demonstrate that fibrillarin is an essential protein in Tetrahymena. The Tetrahymena fibrillarin is expressed but does not complement a NOP1 null mutation when transformed into the yeast Saccharomyces cerevisiae, indicating less functional conservation among fibrillarins than previously suggested.

Effect of structural modification of enol-carboxamide-type nonsteroidal antiinflammatory drugs on COX-2/COX-1 selectivity

Meloxicam (5), an NSAID in the enol-carboxamide class, was developed on the basis of its antiinflammatory activity and relative safety in animal models. In subsequent screening in microsomal assays using human COX-1 and COX-2, we discovered that it possessed a selectivity profile for COX-2 superior to piroxicam and other marketed NSAIDs. We therefore embarked on a study of enol-carboxamide type compounds to determine if COX-2 selectivity and potency could be dramatically improved by structural modification. Substitution at the 6- and 7-positions of the 4-oxo-1,2-benzothiazine-3-carboxamide, alteration of the N-methyl substituent, and amide modification were all examined. In addition we explored several related systems including the isomeric 3-oxo-1,2-benzothiazine-4-carboxamides, thienothiazines, indolothizines, benzothienothiazines, naphthothiazines, and 1,3- and 1,4-dioxoisoquinolines. While a few examples were found with greater potency in the COX-2 assay, no compound tested had a better COX-2/COX-1 selectivity profile than that of 5.

Comparative, behavioural and electrocortical effects of tumor necrosis factor-alpha and interleukin-1 microinjected into the locus coeruleus of rat

The behavioural and electrocortical (ECoG) effects of human recombinant tumor necrosis factor-alpha (hrTNF-alpha) and various forms of interleukin-1 (IL-1) microinjected into the locus coeruleus (LC) of rats were studied. IL-1 induced a typical, dose-dependent, behavioural sedation and/or sleep which was associated with ECoG synchronization. IL-1 beta appeared more potent than IL-1 alpha. During sleep induced by the various forms of IL-1 a dose-dependent increase in total voltage power (0.25-16 Hz) as well as in the 3-6, 6-9 and sometimes 0.25-3 Hz frequency bands was observed. The behavioural and ECoG effects of IL-1 beta were blocked in rats pretreated with anti-IL-1 monoclonal antibodies. The microinjection of hrTNF-alpha into the LC produced a typical pattern characterized by a first short lasting (20-30 min) phase of behavioural arousal and ECoG desynchronization, followed by a longer lasting (45-80 min) phase of behavioural sedation and/or sleep and ECoG synchronization characterized by an increase in total voltage power as well as in the 3-6, 6-9 and sometimes 0.25-3 Hz frequency bands. The behavioural and ECoG effects of hrTNF-alpha were antagonized by a pretreatment (15 min before) with specific anti-TNF-alpha polyclonal antibodies. In addition, a pretreatment with anti-IL-1 receptor monoclonal antibodies was unable to significantly affect the stimulation of behaviour and ECoG desynchronization effects elicited by hrTNF-alpha whilst the same pretreatment completely prevent the sedative and ECoG synchronizing phase elicited by the microinjection of hrTNF-alpha into the LC. These results are consisted with the hypothesis that the sedative and/or soporific behavioural and ECoG changes of hrTNF-alpha are mediated, at LC level, through a local IL-1 release.

A new hepatitis C virus-like flavivirus in patients with cryptogenic liver disease associated with elevated GGT and alkaline phosphatase serum levels

The intriguing co-infection of two flaviviruses (GBV-A and GBV-B) in tamarins and the recent discovery of another flavivirus (GBV-C/HGV) in humans raises the question of the relations between hepatitis C virus (HCV) and GBV-C/HGV. To address this issue the sera of 285 patients with liver disease (102 patients with cryptogenic and 183 with known forms of chronic liver disease) and 19 patients without liver disease were tested for HGV-RNA. GBV-C/HGV-RNA was detected by RT-PCR using primers encompassing 5'NC and NS5 regions and hybridization with specific biotinilated and radiolabelled probes. GBV-C/HGV RNA was found in 11 of 20 (55%) acute hepatitis C patients, in 13 of 117 (11.1%) patients with chronic hepatitis C, in 11 of 27 patients with a liver transplant (40.7%), one of 19 (5.3%) patients with chronic HBV infection, 15 out of 102 (14.7%) patients with cryptogenic liver disease and two out of 19 patients with inflammatory bowel disease. In cryptogenic patients, elevated serum gammaglutamyl transpeptidase (GGT, higher than twice the normal values) and alkaline phosphatase (ALP, above normal values) levels were significantly associated with GBV-C/HGV-RNA infection (P < 0.001). In conclusion GBV-C/HGV appears to be transmitted in humans by blood exposure and to be associated with liver disease in HCV co-infected patients and in a minority of patients with cryptogenic disease. The virus is only occasionally pathogenic for the liver and when liver damage is present; the association with the combined elevation of GGT and APH serum levels might represent a specific feature of the liver tropism of the agent.

Comparison of film-screen combination and digital fluorography in gastrointestinal barium examinations in a clinical setting

OBJECTIVE

Digital fluoroscopy systems have been developed for gastrointestinal barium studies. We compare the performance of a commercially available digital fluoroscopy system with conventional film-screen combinations in gastrointestinal barium studies.

METHODS

Two consecutive groups of outpatients underwent diagnostic barium examinations of esophagus, stomach, or colon using either a film-screen technique (n = 32) or a digital fluoroscopy system with an 8-bit gray scale (n = 31). A standardized biphasic protocol was employed in all patients by the same set of investigators and scored subjectively for contrast performance in single-contrast and double-contrast mode and for spatial resolution.

RESULTS

For the whole group there were no significant differences between the two techniques. In the subgroup of esophagus studies, digital fluoroscopy performed better during the full-column part of the examination. In the subgroup of colon studies, the film-screen technique performed better during the double-contrast part of the examination. To reach sufficient spatial resolution with the digital system (884(2) matrix), the smallest input screen had to be used (23 cm), resulting in a higher number of exposures than with the film-screen combination (P = 0.006). The average amount of film used was not significantly different.

CONCLUSION

In a clinical setting the contrast performance and spatial resolution of digital fluoroscopy systems can be comparable with conventional film-screen combinations.

Validity in nulliparas of increased beta-human chorionic gonadotrophin at mid-term for predicting pregnancy-induced hypertension complicated with proteinuria and intrauterine growth retardation

The objective of the present study was to investigate whether increased beta-human chorionic gonadotrophin (beta HCG) plasma concentrations in an unselected population of nulliparas could predict the occurrence of complicated pregnancy-induced hypertension (PIH). The design was that of a prospective population study. It was conducted at the obstetric departments of Amiens University Hospital and Creil General Hospital on 434 consecutive nulliparas with singleton pregnancies after natural fertilization who accepted the systematic offer of trisomy 21 screening but for whom this disorder was finally estimated. Measurement of plasma concentration of beta HCG (ELISA method) was carried out between 14 and 20 weeks (mean: 17 weeks) of amenorrhea, and measurement of blood pressure and proteinuria (> 300 mg/24 h or Albustix +2) during the first, second and third term and 2-3 months after the delivery, as well as measurement of birth weight for determination of small for gestational age (SGA) babies, 37 women developed PIH, 10 without other complication, 16 with proteinuria (5 of which with SGA babies) and 11 with SGA babies. Furthermore 2 patients presented abruptio placentae without PIH. 395 women did not develop PIH including 389 normotensive women and 6 chronic hypertensive patients without superimposed toxemia. Only 1 was diabetic. None had chronic renal disease. Mean (+/- SD) levels of beta HCG were higher in PIH than in controls: 46,805 +/- 19,068 versus 23,479 +/- 13,463 IU. A pathologic threshold was chosen as the mean for the whole population + 1 SD: 25,613 + 15,479 = 41,082 IU. Elevated levels (above this value) were significantly associated with isolated PIH or PIH complicated with proteinuria and/or with SGA babies. The positive predictive value of this criterion was respectively 11, 15 and 12% for each of these complications. The relative risk (and 95% confidence limit) of women with elevated beta HCG for each of these complications was 20 (6-79), 11 (4-43) and 22 (7-93). Elevated plasma beta HCG found around 17 weeks of amenorrhea predicts PIH complicated with either proteinuria or SGA babies with a positive predictive value comparable to that of the best and earliest test proposed up to now to select nulliparas at high risk of preeclampsia, namely the abnormalities of the Doppler waveforms of the uterine arteries. Since this test is simpler to perform, it represents the most convenient method to screen a population of nulliparas for evaluation of the benefits of low-dose aspirin.

[The sensitivity of Salmonella strains in diarrheal disease to new quinolones compared with other antimicrobial substances]

The sensitivity of 59 Salmonella strains isolated in children with acute diarrhoea was tested against the new quinolones like: Ciproflaxicin (CIP), Norfloxacin (NOR) and Ofloxacin (OFX), as compared to the sensitivity against same aminosides: Gentamicin (GM), Amikacin (AN) against cephalosporins: Ceftazidime (CAZ), Cefalotine (CF) and other currently used antimicrobial agents: Tetraciclin (T), Ampicilin (A), Cloramfenicol (C), Furazolidon (FU). The majority of the studied Salmonella strains, 43 out of 59 strains, belonged to the serotype typhimurium, the most frequently serotype isolated in our geographical area. A very high percentage of Salmonella strains were sensitive against the three quinolones: 98,30% sensitive against NOR, 91,5% sensitive against OFX, 91,50% sensitive against CIP and 96,6% sensitive against AN. In contrast, the Salmonella strains sensitivity was lower in the other tested antimicrobial substances: C (32,2% sensitive strains), GM (8,5%), A (16,9%), CF (11,9%), T (3,4%), FU (1,7%). Out of 59 strains, 45 where resistant to more than four antibiotics, the most often observed pattern was: A, CAZ, CF, GM, T, C, FU.

[The virulence of E. coli strains isolated in urinary infections]

E. coli has been considered to be the most common pathogenic agent associated with the urinary tract infections. The pathogenic features of E. coli strains mainly the adherence, hemolysine and cytotoxine production (CNF) correlated with the affiliation to a specific serological type, provided a subject of active research. In the present study 104 strains of E. coli isolated in patients with urinary tract infections (hospitalised children and adults and outpatient children) were tested for their pathogenic features. The pyelonephritogenic capacity was assessed by the manose-resistant haemagglutination (HAMR) tested with human erythrocytes in 42.30% of strains. The haemolysine production was found in a significant number of strains (32.60%; 38.20% of this percentage enhanced the strains virulence by toxin elaboration (CNF)). The haemolysine production was mostly observed in the strains isolated in hospitalised children while the HAMR was more frequent in outpatient children. The increased incidence of the O6 serogroup among the uropathogenic E. coli strains (41.40%) was mainly recorded in hospitalised children; this finding indicates the O6 serogroup circulation within the health care units. The determination of the pathogenic features, chiefly of the pyelonephritogenic ones has been very useful for the disease prognosis and treatment.

Pyrithiamine-induced thiamine deficiency impairs object recognition in rats

Pyrithiamine-induced thiamine deficiency (PTD) in rats is used to model the etiology, diencephalic neuropathology, and memory deficits of Korsakoff's amnesia. We assessed the performance of rats exposed to PTD on a test of object recognition--nonrecurring-items delayed nonmatching-to-sample (DNMS). PTD produced thalamic lesions similar to those of Korsakoff's amnesics and similar to those previously observed in PTD rats. PTD rats required more trials to master DNMS at a 4-s retention delay than did controls, and after they had done so, they performed more poorly than controls at delays of 15, 30, 60, and 120 s. DNMS deficits were also observed in PTD rats that received training prior to PTD treatment. These findings support the validity of the PTD rat model of Korsakoff's disease by demonstrating that PTD rats display object-recognition deficits that are similar to those reported in Korsakoff amnesics.

Novel non-nucleoside inhibitors of human immunodeficiency virus type 1 reverse transcriptase. 5. 4-Substituted and 2,4-disubstituted analogs of nevirapine

Molecular modeling analysis of the recently published X-ray crystal structure of nevirapine bound to wild type human immunodeficiency virus type 1 reverse transcriptase (WT-RT) indicated the presence of a lipophilic cavity proximal to the 4-position of the inhibitor. A series of 4-substituted derivatives of nevirapine were thus synthesized to assess structure-activity relationships (SARs) and to see if increased binding to this region might translate into greater activity against mutant RTs. The results show that compounds with an appropriately spaced aryl ring appended to the 4-position of the dipyridodiazepinone ring system show good activity against WT-RT. Furthermore certain derivatives appear to inhibit the Y181C mutant RT. Attempts to combine these results with the recent discovery that 2-substituents enhance activity against the Y181C mutant led to a few compounds with moderate activity against both enzymes. The SAR of these two positions, however, could not be combined in a simple fashion.

Management of the hot thyroid nodule

BACKGROUND

Solitary hyperfunctioning nodules of the thyroid gland are usually viewed as benign. They may present with autonomous euthyroidism but are of concern for potential progression to hyperthyroidism. Various methods of treatment are worthy of consideration.

PATIENTS AND METHODS

Forty-five patients with solitary hot thyroid nodules verified by radioisotope scintiscanning were selected for treatment. Thirty-one underwent surgery, usually partial thyroidectomy. Eight euthyroid patients received no treatment, 5 underwent therapy with radioactive iodine (RAI), and 1 received thyroid suppression treatment. The cases were assessed retrospectively.

RESULTS

Thyroidectomy patients had no morbidity, were well, and showed 1 Hürthle cell tumor and 5 coincidental small malignancies associated with benign hot nodules, including a contralateral cancer. Untreated patients showed continuance of good health, but nodules persisted and 1 Graves' orbititis occurred. The RAI-treated patients had persistent nodularity, improved function, and 1 case of hyperparathyroidism. Thyroid feeding only caused iatrogenic toxicity and was discontinued.

CONCLUSIONS

There are various techniques for managing the hot nodule. Nonsurgical methods may be effective, but can result in persistent nodularity and iatrogenic sequelae. Excision had no morbidity in this series and was effective in providing immediate relief of problems present and potential.

Inactivation of cell surface receptors by pheophorbide a, a green pigment isolated from Psychotria acuminata

The inhibition of cytokine and monoclonal antibody binding cell surfaces caused by an extract of Psychotria acuminata, a medicinal plant used in the traditional medicine of the people of Belize (Central Africa), was attributed to the presence of pheophorbide a and pyropheophorbide a. Since the binding of tumor necrosis factor-alpha, interleukin-8, complement factor 5a as well as epidermal growth factor to target cells was dramatically reduced, the inhibition was not receptor or cytokine specific. In addition, the respective binding of radiolabeled monoclonal antibodies CL203 and R15.7 to the cell surface antigens intracellular cell adhesion molecule-1 and lymphocyte function-associated antigen-1 beta-chain was decreased by pretreatment of cells with pheophorbide a as well. In all cases, the inhibition by pheophorbides was dependent on the simultaneous presence of light, indicating causative involvement of a photodynamic process. These observations are not unique to pheophorbides and can be extended to porphyrins as well as to other photodynamic agents. Cytotoxicity resulting from photodynamic therapy (PDT) has been documented by many studies. Our investigations suggest that the inactivation of cell surface receptors contributes not only to an antitumor effect of PDT but also to the systemic immunosuppression, a serious side effect of PDT.

Interaction of EF-C/RFX-1 with the inverted repeat of viral enhancer regions is required for transactivation

The hepatitis B virus (HBV) and polyomavirus (Py) enhancer regions contain multiple cis-acting elements that contribute to enhancer activity. The EF-C binding site was previously shown to be an important functional component of each enhancer region. EF-C is a ubiquitous binding activity that interacts with an inverted repeat sequence in the HBV and Py enhancer regions. Although the EF-C binding site is required for optimal enhancer function, the EF-C site does not possess intrinsic enhancer activity when assayed in the absence of flanking elements. With both the HBV and Py enhancer regions, EF-C stimulates the activity of adjacent enhancer elements in a synergistic manner. EF-C corresponds to RFX-1, a protein that binds to a conserved and functionally important site in major histocompatibility complex (MHC) class II antigen promoter regions. Interestingly, the RFX-1 binding site in MHC class II promoters only contains an EF-C half-site, maintaining one arm of the inverted repeat in an EF-C binding site. We have investigated the binding of purified EF-C and RFX-1 to sites in the Py and HBV enhancer regions that carry mutations that either disrupt one arm of the EF-C inverted repeat, or alter the spacing between the repeats. Our results show that the interaction of EF-C and RFX-1 with an intact inverted repeat is required for functional activity of these viral enhancer regions. Chemical footprinting and modification interference assays show that the interaction of EF-C and RFX-1 with the DRA MHC class II promoter truly represents half-site interaction, and that this binding is unstable. In contrast, the binding of EF-C and RFX-1 to the viral inverted repeats is stable. These results suggest that an additional activity may be required to stabilize EF-C/RFX-1 interaction with the MHC class II promoter, and that viral enhancer regions have evolved high affinity binding sites to sequester dimeric EF-C/RFX-1.