39thInterscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) San Francisco, USA, 26–29 September 1999

CONFERENCE REPORT 39th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) San Francisco, USA, 26–29 September 1999 Nafsika H. Georgopapadakou DuPont Pharmaceuticals Research Labs,Wilmington, Delaware, USA

This major international antimicrobial conference attracted approximately 14 000 delegates from industry and academia and covered a broad range of topics in almost 2500 oral and poster presentations. Summarized here are highlights on microbial resistance and agents in clinical development. BACTERIAL RESISTANCE Drug efflux ilton Saier (U. California, San Diego, USA) gave a crisp overview of efflux pumps. He updated the four (super)families of transporters relevant to drug export previously reported (FASEB J 1998 12: 265–274), and added two new families bringing the total to six:

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The ubiquitous ABC-Binding Casette (ABC) superfamily. It now includes 44 families, seven of which export drugs or drug conjugates. Of these, three are restricted to eukaryotes and four to prokaryotes.The best characterized member is the mammalian MDR. The ubiquitous Major Facilitator (MF) superfamily. It now includes 25 families, some restricted to eukaryotes and others to prokaryotes.At least five of these export drugs.The best characterized member is the bacterial TetB. The now ubiquitous Resistance-Nodulation-Division (RND) superfamily. It includes seven families, three of which efflux drugs in bacteria.The best characterized members are AcrB and MexB. The prokaryotic-specific, Small Multidrug Resistance (SMR) superfamily. It is now includes many new drug efflux pumps.The best characterized members are Smr and EmrE. The novel, prokaryotic-specific, Multi-Antimicrobial Extrusion (MATE) family. Its members resemble topologically those of the MF superfamily but have no similarity in sequence or motifs. The novel, eukaryotic-specific, Multidrug Endosomal Transporter family (MET). It is involved in the transport of nucleosides and steroid hormones.

Hiroshi Nikaido (U. California, Berkeley, USA) reviewed multi-drug efflux in bacteria and its role in antibiotic resis-

tance.The widely distributed SMR and MFS pumps extrude a limited range of compounds, most often cationic detergents such as disinfectants (as in the plasmid-coded Qac pumps of Staphylococcus aureus). In contrast, RND pumps are restricted to gram -negative bacteria (as in MexAB-OprM of Pseudomonas aeruginosa and AcrAB-TolC of Escherichia coli), extrude a wide variety of compounds and are usually coded by chromosomal genes. For example, the AcrAB system pumps out most of the common antibiotics, dyes, detergents and even simple organic solvents. RND pumps occur as multiprotein complexes spanning the outer and the inner membranes, and pump out drugs directly into the external medium rather than into the periplasm. Basal expression of these pumps is responsible for the well-known intrinsic resistance of gram-negative bacteria to amphiphilic and lipophilic drugs, as shown by the lowering of the cloxacillin MIC for E. coli from >512 to 2 µg/ml after disruption of the AcrAB gene. Furthermore, their expression is increased by various environmental signals and by mutations in regulatory genes. Paradoxically, such mutations might be selected for by agents for which traditional resistance mechanisms (modification of the drug or the target) are not readily available, such as disinfectants, quinolones and β-lactamase-stable β-lactams. Efflux inhibitors for resistance? MC-207,110 (Microcide), a dipeptide amide with marginal antibacterial activity (MIC=256 mg/ml), potentiated the activity of levofloxacin against P. aeruginosa 8-fold at 10 µg/ml.The clinical utility of efflux inhibitors as combination therapy with quinolones is unclear since only low-level quinolone resistance is associated with alterations in efflux pumps. High-level resistance is associated with altered DNA gyrase and topoisomerase IV. UK 57,562 (Pfizer) is a tetracycline-efflux inhibitor identified in a high throughput screen using TetC as target. The compound, an indole derivative, reduced the MIC of tetracycline against E. coli strains carrying TetA,Tet B,Tet C,Tet D,Tet K and Tet L resistance determinants 4- to 8-fold at 25 µg/ml (MIC≥50 µg/ml). Streptococcus pneumoniae resistance Macrolides. Currently, about 30% of S. pneumoniae strains are resistant to macrolides, though they remain sensitive to amoxicillin-clavulanic acid and to the newer quinolones such as moxifloxacin. In bacteria, macrolide resistance generally occurs by three mechanisms: target modification (methylation of 23S rRNA; MLS resistance), drug modification (hydrolysis of the lactone macrocycle and phosphorylation of the 2′-hydroxyl group of the amino sugar) and drug efflux mediated by ABC-type transporters. In S. pneumoniae, resistance occurs by two mechanisms: RNA modification and drug efflux. J. Sutcliffe (Pfizer) described a new mechanism of clinical resistance in S. pneumoniae involving modifications in the L4 ribosomal protein. Clinical isolates from Bulgaria, Slovakia and Poland, had a three-amino-acid substitution in a highly conserved region of L4, while clinical isolates from the US had a six-amino-acid insertion in the same region. Everninomycins. Ziracin (SCH 27899), an antibiotic active exclusively against gram-positive bacteria, inhibits protein synthesis by binding to a single site on the 50S ribosomal  1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 335–340

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Georgopapadakou subunit (Kd ~0.1 µM). Several laboratory mutants of S. pneumoniae spontaneously resistant to ziracin were isolated and four that did not have previously identified mutations in L16 were further analyzed.They were found to have mutations in the 23S rRNA (A2469C, C2480T, G2535A, and G2536C) and to lack cross resistance against a spectrum of protein synthesis inhibitors, suggesting that the binding site of ziracin might be unique. S. aureus resistance Macrolides. A macrolide-resistant S. aureus strain carried an energy-dependent efflux system for 14- (erythromycin/clar ithromycin/ro xithromycin/dir ithromycin) and 15- (azithromycin) membered macrolides. The strain also hydrolyzed the macrocyclic lactone ring of 14- and 16(spiramycin) membered macrolides. Genetic analysis of the resistant strain revealed that at least one of the resistance determinants, msrA, was on a 28-kb plasmid and encoded an ABC-type transporter. GE2270A. This is a thiazolyl peptide inhibitor of EF-Tu highly active against gram-positive bacteria. S. aureus exposed to concentrations of GE2270A 4- to 10-fold higher than MICs became resistant at a frequency of 10–8. Mutants displayed a high (MIC>64 µg/ml) or a moderate (MIC=4 µg/ml) level of resistance (MIC of parent=0.03 µg/ml). High-level resistance was associated with a G276S mutation while moderate-level resistance with a G258C mutation in the tufA gene. Both mutations were in a highly conserved domain of EF-Tu. Everminomycins. Two ziracin-resistant mutants of S. aureus, generated by chemical mutagenesis, had 6- to 8-fold reduced binding of ziracin to ribosomes and accumulated significantly less antibiotic. This suggests that accumulation may be linked to binding to ribosomes; that is, ribosomes act as a sink for the antibiotic. Sequencing the rplP locus revealed that in both strains arginine 51 had been mutated to histidine or cysteine. Enterococcus resistance Oxazolidinones. After treating 2 000 patients with predominantly gram-positive infections (staphylococci, enterococci, pneumoccocci), two incidences of linezolid resistance, both in Enterococcus faecium, were reported.As with laboratory resistance, clinical esistance to linezolid was associated with mutations in the 23S rRNA. Perhaps significantly, only 169 of the 200 patients were infected with E. faecium, an organism against which linezolid is not highly active (MIC, 2 µg/ml). Mycobacterium tuberculosis resistance Macrolides. In a late-breaker session, drug tolerance to isoniazid plus ethanbutol and rifampin was reported to be associated with mycobacterial persistence and relapse. Tolerance may thus be an important determinant of the outcome of tuberculosis therapy, although it can arise from drug-nonspecific mechanisms. ANTIBACTERIAL AGENTS IN DEVELOPMENT β-Lactams Cephalosporins. Ceftidoren pivoxil (ME 1207, ME 1206A; TAP Pharmaceutocals) is an orally active aminothiazolyl 336

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cephalosporin, formulated as an ester, highly active against major respiratory pathogens. It is currently in phase III. Carbapenems. E-1010 (Eisai) is a broad-spectrum 1 βmethyl carbapenem currently in phase I. It is active against MRSA, P. aeruginosa and anaerobes. Two additional, orally active, 1 β-methyl carbapenems, CS-834 (Sankyo) and L-084 (Lederle), are in phase II. J-110441 (Banyu/Merck) is a carbapenem antibiotic with potent, broad-spectrum antibacterial activity. It also has inhibitory activity at sub-micromolar concentrations against class B metallo-β-lactamases (IMP-1), and class A and C serine β-lactamases. Glycopeptides LY 333328 (Lilly) is a semisynthetic derivative of vancomycin active against vancomycin-resistant Enterococci. It is currently in phase III. It has more rapid bactericidal activity compared to vancomycin against multiply resistant S. pneumoniae in vitro. Quinolones William Craig (University of Wisconsin) gave an overview of the pharmacodynamics and pharmacokinetics of quinolones that are major determinants of their efficacy. For quinolones to be effective, AUC/MIC must be >25. Protein binding also affects activity; it is increased in gemifloxacin and decreased in sitafloxacin (free drug is the same). None of the quinolones reach adequate levels for activity against all strains of Pseudomonas.This organism, and methicillin-resistant Staphylococci, are the weak segments of the otherwise impressive antibacterial spectrum of quinolones. There are at least five quinolones in clinical development; Avelox (avalox, moxifloxacin, BAY- 128039) and clinafloxacin (PD 127391, CI960, AM 1091; Warner Lambert) submitted to FDA; Factive (gemifloxacin, SB 265805, LB20304; SmithKline Beecham) and sitafloxacin (DU 6859A; Daiichi), in phase III; and Tequin (gatifloxacin, BMS 284756, AM 1155; Bristol-Myers Squibb) in phase I.They are generally more potent against S. pneumoniae and anaerobes than available quinolones and, those with a C-8 methoxy group, have improved activity against quinolone-resistant S. aureus. However, they are only moderately active against P. aeruginosa. With the exception of sitafloxacin which has bid dosing, all others have qd dosing. Ketolides Telithromycin (HMR 3647; Aventis) has completed phase III clinical trials and is the ketolide (3-keto macrolide) furthest along in clinical development. It has potent activity against respiratory pathogens, both common (S. pneumoniae (though not constitutively macrolide-resistant strains), Hemophilus influenzae, Moraxella catarrhalis) and atypical (Mycoplasma, Legionella, Chlamydia pneumoniae). It does not induce MLSB resistance, is stable in acid (pH 1), has oral bioavailability, low potential for drug-drug interactions (reduced CYP3A induction relative to erythromycin), excellent tissue penetration and long serum half-life permitting once-a-day administration. ABT-773 (Abbott), is currently in phase II. In vitro, it had MIC90s of 0.12 and 0.5 µg/ml against methicillin-resistant

IC AAC S. pneumoniae and Streptococcus pyogenes, respectively. It was highly active (MICs <1 µg/ml) against both typical and atypical respiratory pathogens with the exception of H. influenzae (MIC=4 µg/ml). Glycylcyclines These are semisynthetic tetracyclines with broad-spectrum antibacterial activity (gram-positive, -negative, anaerobes), including activity against strains resistant to traditional tetracyclines through ribosomal protection (tet(M) and tet(O) determinants) or efflux (tet(A), tet(B), tet(C) and tet(K) determinants). GAR-936 (Wyeth-Ayerst) is a 9-t-butylglycylamido derivative of minocycline with an activity spectrum typical for members of this group (MIC90s against MRSA,VRE and penicillin-resistant P. pneumoniae ≤0.5 µg/ml). GAR-936 is currently in phase II studies. Oxazolidinones Linezolid (PNU-10076, Zyvox; Pharmacia and Upjohn) is a synthetic agent with activity predominantly against grampositive bacteria. It is currently in phase III studies both as an oral and iv formulation. The compound acts primarily additively with other antibacterial drugs. It does not appear to be effective against meningitis, most likely due to low CSF. G. Eliopoulos (Beth Israel Deaconess Hospital, Boston, USA) presented phase III data on the use of linezolid in patients with suspected soft-tissue gram-positive infections. Other compounds SCH-27899 (ziracin; Schering Plough) is an intravenous oligosaccharide antibiotic of the everninomycin class active against gram-positive bacteria, including intracellular bacteria such as Legionella pneumophila. It is currently in phase III. SB 247386 and SB 268091 (SmithKline Beecham) are two semisynthetic pleuromutilins that are being developed as topical agents. They have excellent in vitro activity against gram-positive bacteria normally associated with skin and skin structure infections (S. aureus, Staphylococcus epidermidis, S. pyogenes). MICs were ≤1 µg/ml for SB 247386 and ≤0.06 µg/ml for SB 268091. Both compounds were more active than mupirocin in a murine surgical wound infection model. Bactolysin (MBI 226) is a synthetic, cationic antimicrobial peptide. It was found to be highly active in treating ophthalmic infections in a rabbit eye model. In phase I trials, it was shown to prevent colonization of intravenous catheters. It has received fast-track designation from FDA for the prevention of central venous catheter-related bloodstream infections. Broad- or narrow-spectrum antibacterials? R. Moellering gave a balanced, multifaceted perspective on the perennial issue of antibacterial spectrum. Determinants of antibacterial spectrum are: the biochemical target; the drug’s ability to reach the target (for example, vancomycin can not go through the outer membrane and is thus inactive against gram-negative bacteria; aminoglycoside uptake involves active transport which is not possible in anaerobes); the effect of resistance development (for example, resistance

has narrowed the spectrum of β-lactams and tetracyclines); and the concentration of antimicrobial at the target host site. Advantages of broad-spectrum antibiotics include empirical treatment (less need for diagnostics) and ability to treat polymicrobial infections such as sepsis and diabetic foot. Disadvantages include greater impact on normal flora (elimination of vitamin K producers, superinfection), resistance selection in multiple organisms and false sense of security. Advantages of narrow-spectrum antibacterials are preservation of normal flora in patients (thereby decreasing the risk for superinfection) and perhaps decreased likelihood for selection of resistant organisms. Disadvantages include the requirement for precise (and rapid) diagnosis and the lack of utility in polymicrobial infections. Currently, narrow-spectrum drug development (glycylcyclines, oxazolidinones) is driven by the emergence of multiresistant organisms (MRSA and to a lesser extent VRE). FUNGAL RESISTANCE Drug efflux Elisabetta Balzi (Catholic U. Louvain, Belgium) reviewed efflux pumps in fungi, focusing primarily on the model, nonpathogenic yeast Saccharomyces cerevisiae and secondarily on the most common fungal pathogen, Candida albicans. Both organisms have developed a multitude of permeases (MF superfamily) and pumps (ABC superfamily) for the efflux of multiple drugs. In addition, S. cerevisiae has revealed a complex network of pleiotropic drug resistance genes (PDR) controlled by the transcription regulators Pdrlp and Pdr3p.Targets of the two transcription regulators, identified through genetic and bioinformatics tools, include ATPdependent and proton-motive-force-dependent permeases as well as genes involved in lipid metabolism and stress response. Her lab generated strains with multiple deletions of up to 8 ABC pumps, 5 MF permeases and 2 PDR regulator genes in different combinations. After screening with these strains 350 compounds that included clinically relevant antifungals, they found that the three major ABC pumps, PDR5, SNQ2 and YOR1, mediate resistance to most compounds.The strains are of obvious value for screening large compound libraries for new leads. Fluconazole resistance is a significant problem in the long-term treatment of recurrent oropharyngeal candidiasis in AIDS patients. Resistance is often due to active drug efflux and correlates with overexpression of the ABC transporters CDR1/CDR2 or the MF transporter MDR1. Two pairs of matched fluconazole-susceptible and resistant isolates in which fluconazole resistance correlated with overexpression of the MDR1 gene were analyzed.The MDR1 promoter was fused to a green fluorescent protein (GFP) gene that was integrated at an ectopic site in the C. albicans genome.The MDR1 promoter was activated in the two resistant isolates but not in the corresponding susceptible isolates suggesting that the constitutive activation of MDR1 is due to mutations in a trans-regulatory factor. Rapid, transient fluconazole resistance in colonizing and invasive C.albicans from bone-marrow transplant patients was observed after in-vitro drug exposure. This phenotype was associated with CDR expression and was ‘hetero-resistant’; that  1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 335–340

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Georgopapadakou is, different substrains had different susceptibilities to fluconazole. CDR1p may mediate the translocation of phosphatidylethanolamine from the inner to the outer membrane leaflet, similar to the ABC transporters Mdr1p, Mdr3p and Mdr2p which also have been shown to possess outwarddirected phospholipid translocase activity. Efflux inhibitors for resistance? Multidrug resistance (MDR) pumps have been implicated in azole resistance of Candida species. At Microcide, chemical and natural product libraries were screened for inhibitors of CDR-type pumps (ABC superfamily) and several inhibitors were identified. They inhibited multiple CDR-type pumps but not MDR pumps of the major facilitator superfamily such as BenR.They also potentiated the activity of fluconazole and SCH 56592 by 64- to 128-fold in C. albicans and 8- to 16-fold in Candida glabrata. MC-510,027 is a natural product lacking significant antifungal activity. It potentiated the activity of fluconazole, itraconazole, SCH56592, and terbinafine against several Candida species (C. albicans, C. glabrata, Candida tropicalis and others) by as much as 1000-fold. ANTIFUNGAL AGENTS IN DEVELOPMENT Azoles Posaconazole (SCH 56592; Schering Plough) is an orally active, metabolically stable derivative of itraconazole currently in phase II/III. In vitro, the compound had comparable activity to voriconazole and itraconazole against several yeast species, C. neoformans and C. immitis. In neutropenic rabbits posaconazole, at concentrations of 6 mg/kg or above, was effective in the treatment and prevention of invasive pulmonary aspergillosis. SCH 59884 is a water-soluble prodrug of SCH 56592 for intravenous formulations in phase I trials. SCH 59884 is a phosphate ester prodrug of SCH 56592, and is hydrolyzed in liver and serum to SCH 56592 via an ester intermediate.The compound is inactive in vitro but active in murine models of pulmonary aspergillosis and systemic candidiasis. Ravuconazole (BMS 207147, ER30346; Bristol-Myers Squibb) is currently in phase II. It was less active than itraconazole in vitro against Aspergillus species. A. flavus appeared to be resistant to both azoles. Glucan synthase inhibitors: echinocandins Caspofungin (Cansidas, MK-991, L-743872, MK 0991; Merck) is an injectable semisynthetic echinocandin derivative currently in phase III. FK-463 (Fujisawa) is a water-soluble, semisynthetic echinocandin currently in phase II. It has in vitro and in vivo activity comparable to caspofungin and an additive effect when combined with amphotericin B. Polyene formulations Nyotran (AR 121;Aronex Pharmaceuticals) is a liposomal formulation of nystatin currently in phase III. Nyotran was evaluated in neutropenic mice infected with an amphotericin B-resistant isolate of A. fumigatus. In that model, the optimal dose for nyotran was 5 mg/kg daily, with 90% survival, and 338

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was superior to lipid formulations of amphotericin on a mg per mg basis. Amphotericin B cochleate (AMBc; Biodelivery Science) is a novel lipid formulation of amphotericin A involving phosphatidylserine and calcium ions.AMBc has a unique multilayered structure consisting of continuous, solid lipid bilayer sheets rolled up in a spiral (hence the name cochleate). AMBc has equivalent activity in vitro and in vivo to amphotericin B (fungizone) and is orally active. NS-718 is a novel lipid nanosphere-encapsulated amphotericin B. It was active in a murine model of pulmonary cryptococcosis and, though amphotericin concentrations in the kidney were higher in the NS-718 animals, nephrotoxicity was lower. VIRAL RESISTANCE Human immunodeficiency virus (HIV) The life cycle of HIV has been intensively studied and a number of stages have been exploited as antiretroviral targets. Among them are processes catalyzed by the three enzymes encoded by HIV: formation of proviral DNA by reverse transcriptase (RT), insertion of proviral DNA into the host genome by integrase and cleavage of the precursor viral proteins by acidic protease. Clinically useful agents developed so far are RT inhibitors (RTIs), which are further divided into nucleoside RTIs (NRTIs) and non-nucleoside RTIs (NNRTIs), and protease inhibitors (PIs). They are the components of the Highly Active Antiretroviral Therapy (HAART), which typically consists of two NRTIs and a PI or an NNRTI. Several compounds of each class are now available commercially: •

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NRTIs: zidovudine (AZT, Retrovir; Glaxo-Wellcome), didanosine (ddI,Videx; Bristol-Myers Squibb) zalcitabine (ddC, Hivid; Roche), lamivudine (3TC, epivir; Glaxo Wellcome), stavudine (d4T, Zerit; Bristol-Myers Squibb), abacavir (ABC, Ziagen; Glaxo Wellcome). PIs: indinavir (IDV, Crixivan; Merck), ritonavir (RTC, Norvir;Abbott), saquinavir (SQ, Invirase/Fortovase; Roche) nelfinavir (NFV,AG 1343,Viracept;Agouron), amprenavir (APV,VX-478,Agenerase;Vertex/Glaxo Wellcome; resistance profile different from other PIs). NNRTIs: nevirapine (NVP,Virammune; Roxane Labs/Boehringer Ingelheim), delavirdine (DLV, Rescriptor; Pharmacia & Upjohn) efavirenz (EFV, Sustiva; DuPont Pharmaceuticals), emivirine (MKC-442; Triangle Pharmaceuticals; in phase III).

HAART has proved particularly challenging because of its demanding dosing schedule, potential side effects (particularly nausea/vomiting) and drug interactions, and the threat of the relatively rapid development of resistance and cross resistance if the regimen is not maintained. HIV-1 RNA <50 copies/ml is the goal, complete eradication being unlikely.To achieve clinical success (defined as >64% patients with HIV1 RNA <400 copies/ml), HAART requires an adherence level of 90% or better. Treatment factors promoting adherence include easier dosing requirements (lower pill burden, no food restrictions, bid or qd regimens), fewer side-effects and drug interactions, and diminished risk for resistance.

IC AAC Additional issues before treatment initiation are pre-existing HIV resistance, variable sensitivity of wild type HIV, and high HIV RNA coupled with low CD4 count. In a late-breaker session, discontinuation of HAART after 3 years of complete HIV suppression (<50 copies/ml) of four newly infected individuals was shown to cause a rebound in plasma viremia and decreased CD4. T cell count. However, rebound appeared later than in chronically infected individuals who stop therapy. In three of four patients partial control of viremia, presumably immunologically mediated, was observed. Target-by-target resistance, Richard D’Aquila (Harvard Medical School, Boston, USA) reviewed new information on HIV resistance: transmission of resistant virus; genomic changes, cross resistance and viral fitness; and evaluation of resistance testing.The first resistance mutations occur in the RT or P gene, increase drug IC50s, and may reduce viral replication capacity (‘fitness’) in the absence of drug. Subsequent resistance mutations may increase fitness, may increase IC50 further, and may create a platform for cross resistance. In the case of PI resistance, additional mutations can occur in the gene for the P substrate (gag-pol), increasing the activity of mutant P. NRTI resistance mutations are clustered in a 65 to 74 amino-acid region (K65R, D67N, T69D, K70R, L74V) and at residue 184 (M to V). Predictably, the oldest NRTI, AZT, has selected for the greatest number of mutations. PI resistance is commonly associated with M46I,L, I54V,L, L63P, A71V,P, V82AFT, 184V, and L90M amino acid substitutions. Some mutations, such as D30N (nelfinavir) and to a lesser degree L90M (saquinavir) substitutions, reduce replicative fitness. However, isolates with multiple mutations, such as M46I/L63P/V82T/184V and L10R/M46I/L63P/V82T/184V (indinavir) are as fit as the wild type suggesting that some mutations are compensatory. Patients who fail nelfinavir regimens may be successfully salvaged with ritonavir and sequinavir. Amprenavir may have limited cross resistance to the other PIs, underlying the benefit of new agents with unique resistance profiles. In a late-breaker session, a clinical study with 71 antiretroviral therapy (ART)-experienced patients (viral load, >5000 copies) starting a new ART regimen sustained virologic suppression was predicted by phenotypic susceptibility to drugs in the new regimen. ART history was a less significant predictor of virologic suppression than phenotypic susceptibility. Genotyping HIV isolates improves salvage regimen choices. In addition, genotypic/phenotypic testing can provide a cost-effective (and toxicity-sparing) approach to determining which drugs in a regimen are failing and help decide which drugs to avoid in a subsequent regimen.This is particularly important given the prevalence of resistance to some older agents, such as the NRTIs AZT and 3TC. NNRT resistance is commonly associated with a K103N amino acid substitution. However, in a clinical study with 23 patients, emivirine resistance was associated with mutation other than K103N in isolates from almost half of the patients (residues 100, 108, 188, 190) leaving them susceptible to at least one additional NNRTI in vitro.

ANTIVIRAL AGENTS IN DEVELOPMENT HIV New agents within the three established classes are active against some resistant isolates, penetrate into cellular and bodily compartments and have improved dosing requirements, toxicity and drug interaction profiles. Nevertheless, many heavily pretreated patients have no remaining therapeutic options. •

NRTIs Emtricitabine (FTC;Triangle) resistance profile similar to 3TC; more potent, qd dosing Lodenosine (FddA; US Bioscience) little in vitro crossresistance with other NRTIS; retains activity in strains with Q151 M; possible qd dosing; in phase II/III BCH-10652 (dOTC; Biochern-Pharma) active against AZT and 3TC-resistant virus in animal models; multiple passages necessary to induce in vitro resistance (L74V and K65R); bid dosing; in phase II DAPD/DXG (Triangle) significant in vitro activity against common resistance mutations (41L, 67N, 184V, 215Y/F, 219Q)

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PIs ABT-378 (Abbott) active against ritonavir and indinavir resistant mutants (codon 82 mutations); possible qd; in phase III Tipranavir (U-140690; Pharmacia & Upjohn) active against most isolates resistant to available PIs. 48 V and 90M mutants hypersensitive, 82A mutants sensitive, 82T and 84 V mutants resistant. Potent inducer of CYP3A system, hence liable to drug-drug interactions; tid dosing; in phase II AG-1776 (Agouron) active against 82A/90M, 82F/90M, 82A, 64 V90M, 48 V/82A, and 30N mutants L-756, 423 (Merck) combining L-756, 423 with indinavir may overcome indinavir resistance; qd dosing GW-433908 (VX-175;Vertex/Glaxo Wellcome) prodrug of amprenavir; in phase I/II BMS-232, 632 (Bristol-Myers Squibb) active against most mutants resistant to available PIs; probable qd dosing; in phase II

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NNRTIs Emivirine (MKC-442;Triangle) NRTI with mechanistic action similar to NNRTI; bid dosing; phase II/III AG-1549 (S-1193; Shionogi/Agouron/Warner Lambert) active against K103N,V106A, L100I mutants; ≥2 NNRTI resistance mutations needed for reduced susceptibility; bid or tid dosing (half-life, 5.7 h); in phase II DMP-961/DMP-963 (DuPont Pharma) 3- to 8-fold greater activity in vitro against K103N mutants than efavirenz; probable qd dosing PNU-142721 (Pharmacia & Upjohn) 50-fold more potent than delaviridine; active at 20 nM in vitro against delaviridine resistant mutants GW420867X (HBY-1293, GW867; Hoechst; Glaxo Wellcome) higher resistance threshold than current NNRTIs; single isolates remain sensitive; can be given

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with or without food; probable qd dosing (half-life, 50 h); in phase II Talviraline (HBY-097; Hoechst, licensed to Glaxo Wellcome) in phase II Nucleotide RTIs Adefovir dipivoxil (Preveon; Gilead Sciences) increased activity against M184V mutants, therefore potentially synergistic with drugs that ccause that mutation; future uncertain after non-approval recommendation by FDA Antiviral Drugs Advisory Committee PMPA (Gilead Sciences) favorable resistance profile; qd dosing Ribonucleotide reductase inhibitors Hydroxyurea (HU, Hydrea/Droxia; Bristol-Myers Squibb) in phase II (bid and qd) in combination with ddI and d4T Chemokine receptor antagonists AMD3100 (AnorMED, Canada) potent, low molecular weight antagonist for CXCR4 (co-receptor for HIV); in phase I/II TAK 779 (Takeda) nanomolar, small molecule antagonist for CCr5; inhibits replication of macrophage-tropic (R5) HIV-1 in human peripheral blood mononuclear cells with an EC50 of 3 nM Fusion inhibitors (peptides) T-20 and T1249 (Trimeris) bind to gp41, a viral protein involved in the fusion of HIV with uninfected cells, and thereby prevent viral replication.T-20, given to patients intravenously at dosages 30 or 100 mg bid as monotherapy, significantly decreased mean HIV levels in all patients. No drug-rel/ted side-effects occurred, other than local site reactions (rash, pain). iv/sc dosing. PRO-542 (Progenics) phase I studies reported. In a single dose study, PRO-542 reduced viral load at 10 mg/kg.The compound is currently in phase II

Cytomegalovirus (CMV) In a late-breaker session, A.P. Limaye (U. Washington, Seattle, USA) reported ganciclovir-resistant CMV in solid organ transplant recipients who failed to respond to iv ganciclovir treatment. BAY 38–4766 (Bayer) is an inhibitor of viral DNA cleavage. In vitro, it is about 5-fold more potent than ganciclovir against several strains. In vivo (rats and dogs), it was orally absorbed with absolute bioavailability 30–50%. In phase I trials, it was tolerated up to 2,000 mg as a single oral dose. Its main metabolite, BAY 43–969, also showed antiviral activity. Enterovirus Enteroviruses (EVs) and rhinoviruses (RVs) are among the most common and important viral pathogens. In the USA alone, Evs cause 5–10 million symptomatic infections annually, including meningitis, encephalitis, myocarditis, hemorrhagic conjuctivitis, severe neonatal disease and non-specific febrile illnesses (Harley Rotbart, U. Virginia School of Medicine, Charlottesville, USA). Pleconaril (ViroPharma) inhibits EV and RV replication by inhibitong viral uncoating and viral attachment to the host cell receptors.The drug has a good safety profile, is metabolically stable, and has high oral bioavailability. S.D. Shafran (U. Alberta, Edmonton, Canada) reported a randomized, placebocontrolled trial of pleconaril in adult patients with confirmed enteroviral meningitis. Pleconaril reduced the time to resolution of headache and symptoms of meningitis by 2 days. Similar results were reported by F. Hayden (U.Virginia, Charlottesville, USA) reported from a randomized trial of pleconaril in adults with acute respiratory illness. AG7088 (Agouron) is a rhinovirus 3C protease inhibitor with in-vitro antirhinoviral activity against clinical isolates superior to that of pleconaril. It is currently in phase I/II studies.

HEPATITIS B VIRUS (HBV) Geoffrey Dusheiko (Royal Free Hospital, London, UK) discused new development in the treatment of HBV infections. Current treatments are limited to interferon alpha, with a 20–40% response rate and lamivudine, which targets HBV DNA polymerase. The use of lamivudine, a component of HAART regimens, in HBV and HIV coinfection is particularly attractive. Although lamivudine is effective in anti-HBe positive patients, relapse rates are relatively high after one year of treatment. Combination of lamivudine and interferon alpha in HBeAg-positive patients is under consideration as are other agents (famciclovir, adefovir, lobucavir). Hepatitis C virus (HCV) The current standard of care is Rebetron (Schering), a combination of ribavirin and interfereon α (Intron-A). Schering is conducting late-stage studies on PEG-Intron, a once-a-week, long-acting form of Intron A for HCV. Vertex presented preliminary data (AASLD) from a three-dose (100 mg, 200 mg, 400 mg), phase IIa trial of VX-497, an inhibitor of IMPDH (inosine monophosphate dehydrogenase). VX-497 works via two mechanisms, one antiviral the other anti-inflammatory, and appears to deplete the nucleotide pool in the human cell that is used by HCV when replicating. 340

Drug Resistance Updates (1999) 2, 335–340

 1999 Harcourt Publishers Ltd

SUMMARY As billion-dollar anti-infectives have reached (acyclovir) or are reaching (ciprofloxacin) the end of their patent lives, new ones are poised to take their place. The new compounds presented at this year’s ICAAC were largely members of known families.The only surprise was the strong presence of anti-HIV compounds. Clearly, antivirals have come of age, not only for HIV, CMV, HCV but also for the less serious viruses such as enterovirus and rhinovirus.

Correspondence to: Nafsika H. Georgopapadakou PhD, DuPont Pharmaceuticals, Experimental Station, E400/3456A, PO Box 80400, DE 19880–0400, USA.Tel: +302 695 8525; Fax: +302 695 7407; E-mail: [email protected]