Cellular pharmacokinetics of telavancin, a novel lipoglycopeptide antibiotic, and analysis of lysosomal changes in cultured eukaryotic cells (J774 mouse macrophages and rat embryonic fibroblasts)

Dual-targeted poly(amino acid) nanoparticles deliver drug combinations on-site: an intracellular synergistic strategy to eliminate intracellular bacteria

Multi-drug combinations are a common strategy for the treatment of intracellular bacterial infections. However, different internalized pathways and the accumulation of the composite drugs at different subcellular organelles very much reduce their efficacy. Herein, an intracellular synergistic strategy is proposed, which is realized by on-site delivery of a drug combination using a macrophage/intracellular bacterium-dual targeted drug delivery system (DDS). The DDS is fabricated by encapsulating vancomycin (Van) and curcumin (Cur) into poly(α-N-acryloyl-phenylalanine)-block-poly(β-N-acryloyl-D-aminoalanine-co-2-O-acetyl-α-D-mannosyloxy) nanoparticles, denoted by (Van + Cur)@F(AM) NPs. Mannose ligands on (Van + Cur)@F(AM) NPs trigger their specific internalization in macrophages, while aminoalanine moieties subsequently drive the NPs to target intracellular methicillin-resistant Staphylococcus aureus (MRSA). Thereafter, Van and Cur are durably released in a synergistic dose at the residence site of intracellular MRSA. Under this intracellular synergistic effect, (Van + Cur)@F(AM) NPs show superior elimination efficiency in vitro and in vivo compared to the control groups, including free Van, (Van + Cur), the DDS encapsulated Van and the DDSs separately-encapsulated Van and Cur. Furthermore, (Van + Cur)@F(AM) NPs significantly enhance the in vivo antibacterial capacity by modulating the immune response. Therefore, this dual-targeted DDS-assisted intracellular synergistic antibacterial strategy of drug combination is an effective therapeutic against intracellular bacteria.

Development and Preclinical Evaluation of New Inhaled Lipoglycopeptides for the Treatment of Persistent Pulmonary Methicillin-Resistant Staphylococcus aureus Infections

Chronic pulmonary methicillin-resistant Staphylococcus aureus (MRSA) disease in cystic fibrosis (CF) has a high probability of recurrence following treatment with standard-of-care antibiotics and represents an area of unmet need associated with reduced life expectancy. We developed a lipoglycopeptide therapy customized for pulmonary delivery that not only demonstrates potent activity against planktonic MRSA, but also against protected colonies of MRSA in biofilms and within cells, the latter of which have been linked to clinical antibiotic failure. A library of next-generation potent lipoglycopeptides was synthesized with an emphasis on attaining superior pharmacokinetics (PK) and pharmacodynamics to similar compounds of their class. Our strategy focused on hydrophobic modification of vancomycin, where ester and amide functionality were included with carbonyl configuration and alkyl length as key variables. Candidates representative of each carbonyl attachment chemistry demonstrated potent activity in vitro, with several compounds being 30 to 60 times more potent than vancomycin. Selected compounds were advanced into in vivo nose-only inhalation PK evaluations in rats, where RV94, a potent lipoglycopeptide that utilizes an inverted amide linker to attach a 10-carbon chain to vancomycin, demonstrated the most favorable lung residence time after inhalation. Further in vitro evaluation of RV94 showed superior activity to vancomycin against an expanded panel of Gram-positive organisms, cellular accumulation and efficacy against intracellular MRSA, and MRSA biofilm killing. Moreover, in vivo efficacy of inhaled nebulized RV94 in a 48 h acute model of pulmonary MRSA (USA300) infection in neutropenic rats demonstrated statistically significant antibacterial activity that was superior to inhaled vancomycin.

The lysosome: A potential juncture between SARS-CoV-2 infectivity and Niemann-Pick disease type C, with therapeutic implications

Drug repurposing is potentially the fastest available option in the race to identify safe and efficacious drugs that can be used to prevent and/or treat COVID‐19. By describing the life cycle of the newly emergent coronavirus, SARS‐CoV‐2, in light of emerging data on the therapeutic efficacy of various repurposed antimicrobials undergoing testing against the virus, we highlight in this review a possible mechanistic convergence between some of these tested compounds. Specifically, we propose that the lysosomotropic effects of hydroxychloroquine and several other drugs undergoing testing may be responsible for their demonstrated in vitro antiviral activities against COVID‐19. Moreover, we propose that Niemann‐Pick disease type C (NPC), a lysosomal storage disorder, may provide new insights into potential future therapeutic targets for SARS‐CoV‐2, by highlighting key established features of the disorder that together result in an “unfavorable” host cellular environment that may interfere with viral propagation. Our reasoning evolves from previous biochemical and cell biology findings related to NPC, coupled with the rapidly evolving data on COVID‐19. Our overall aim is to suggest that pharmacological interventions targeting lysosomal function in general, and those particularly capable of reversibly inducing transient NPC‐like cellular and biochemical phenotypes, constitute plausible mechanisms that could be used to therapeutically target COVID‐19.

Clinical Pharmacokinetics and Pharmacodynamics of Telavancin Compared with the Other Glycopeptides

Telavancin was discovered by modifying the chemical structure of vancomycin and belongs to the group of lipoglycopeptides. It employs its antimicrobial potential through two distinct mechanisms of action: inhibition of bacterial cell wall synthesis and induction of bacterial membrane depolarization and permeabilization. In this article we review the clinically relevant pharmacokinetic and pharmacodynamic data of telavancin. For comparison, the pharmacokinetic and pharmacodynamic data of the other glycopeptides are presented. Although, in contrast to the newer lipoglycopeptides, telavancin demonstrates a relatively short half-life and rapid total clearance, its apparent volume of distribution (V_d) is almost identical to that of dalbavancin. The accumulation of telavancin after repeated dosing is only marginal, whereas the pharmacokinetic values of the other glycopeptides show much greater differences after administration of multiple doses. Despite its high plasma-protein binding of 90% and relatively low V_d of approximately 11 L, telavancin shows near complete equilibration of the free fraction in plasma with soft tissue. The ratio of the area under the plasma concentration-time curve from time zero to 24 h (AUC₂₄) of unbound plasma concentrations to the minimal inhibitory concentration (MIC) required to inhibit growth of 90% of organisms (MIC₉₀) of Staphylococcus aureus and S. epidermidis of telavancin are sufficiently high to achieve pharmacokinetic/pharmacodynamic targets indicative for optimal bacterial killing. Considering both the AUC₂₄/MIC ratios of telavancin and the near complete equilibration of the free fraction in plasma with soft tissue, telavancin is an appropriate antimicrobial agent to treat soft tissue infections caused by Gram-positive pathogens. Although the penetration of telavancin into epithelial lining fluid (ELF) requires further investigations, the AUC₂₄/MIC ratio for S. aureus indicates that bactericidal activity in the ELF could be expected.

Semi-Mechanism-Based Pharmacodynamic Model for the Anti-Inflammatory Effect of Baicalein in LPS-Stimulated RAW264.7 Macrophages

Monitoring of the inhibition of TNF-α, IL-6, iNOS, and NO is used to effectively evaluate anti-inflammatory drugs. Baicalein was found to have good anti-inflammatory activities, but its detailed cellular pharmacodynamic events have not been expatiated by any other study. The inflammatory mediators, including TNF-α, IL-6, iNOS, and NO production in RAW264.7 macrophage induced by LPS, were measured. It was found that these data showed a sequential pattern on time and based on these points a cellular pharmacodynamic model was developed and tested. TNF-α and IL-6 were quantified by ELISA, NO was detected by Griess and iNOS expression was measured by Western blot. The pharmacodynamic model was developed using a NLME modeling program Monolix® 2016R1.¹The results showed that baicalein quickly suppressed release of TNF-α in a concentration-dependent manner, and consequently causing the diminution of IL-6 and iNOS/NO. The pharmacodynamic model simulation successfully described the experimental data, supporting the hypothesis that IL-6 and iNOS /NO release after LPS stimulation is mediated by TNF-α rather than LPS directly. The pharmacodynamic model allowed a well understanding of the cellular pharmacodynamic mechanism of baicalein in the treatment of inflammatory diseases.

Evaluation of Pharmacodynamic Interactions Between Telavancin and Aztreonam or Piperacillin/Tazobactam Against Pseudomonas aeruginosa, Escherichia coli and Methicillin-Resistant Staphylococcus aureus

Introduction

In clinical trials comparing telavancin (TLV) with vancomycin for treatment of hospital-acquired pneumonia, TLV demonstrated lower clinical cure rates than vancomycin in patients who had mixed gram-positive and -negative infections and were concomitantly treated with either aztreonam (ATM) or piperacillin/tazobactam (PTZ). Here, we investigated therapeutic interactions between TLV and ATM or PTZ in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model under simulated reduced renal function conditions.

Methods

In vitro one-compartment PK/PD models were run over 96 h simulating TLV 10 mg/kg every 48 h, ATM 500 mg every 8 h and PTZ continuous infusion 13.5 g over 24 h alone and in combination against P. aeruginosa, E. coli and methicillin-resistant S. aureus (MRSA). The efficacy of antimicrobials was evaluated by plotting time-kill curves and calculating the reduction in log₁₀ cfu/ml over 96 h.

Results

Against both MRSA strains, TLV was rapidly bactericidal at 4 h and maintained its activity over 96 h with no observed antagonism by either ATM or PTZ. PTZ maintained bacteriostatic and bactericidal activities against E. coli ATCC 25922 and clinical strain R1022 at 96 h, whereas both strains regrew as soon as 24 h in ATM models. Against P. aeruginosa ATCC 27853, regrowth was noted at 24 h in models simulating ATM and PTZ. The addition of TLV to ATM or PTZ had no appreciable impact on activity against the two E. coli strains and P. aeruginosa strain.

Conclusions

The combinations of TLV and either ATM or PTZ did not demonstrate any antagonistic activity. Clinical variables and patient characteristics should be further explored to determine possible reasons for discrepancies in outcomes.

Funding

Theravance Biopharma Antibiotics, Inc.

Intracellular activity of a membrane-active glycopeptide antibiotic against meticillin-resistant Staphylococcus aureus infection

Staphylococcus aureus is a facultative intracellular pathogen and there are limited options for the treatment of severe intracellular bacterial infections. The membrane-active glycopeptide antibiotic Van-QC8 is a permanent positively charged lipophilic vancomycin analogue that demonstrates high activity against clinically relevant drug-resistant Gram-positive bacteria both in vitro and in vivo. In this study, the intracellular activity of Van-QC8 was evaluated against meticillin-resistant S. aureus (MRSA) infection in RAW macrophages. Furthermore, the mechanism of intracellular uptake of Van-QC8 was investigated. Van-QC8 showed time- and concentration-dependent bactericidal activity against intracellular MRSA. Van-QC8 displayed significantly higher intracellular activity compared with vancomycin and linezolid. Cellular uptake of Van-QC8 was found to be through clathrin-dependent and -independent and caveolin-dependent and -independent endocytic pathways. The findings of this study suggest that Van-QC8 could be translated clinically for the treatment of intracellular infections due to MRSA.

Quantitative determination of telavancin in pregnant baboon plasma by solid-phase extraction and LC-ESI-MS

The increasing incidence and severity of methicillin- and vancomycin-resistant infections during pregnancy prompted further development of telavancin. The understanding of the pharmacokinetics of telavancin during pregnancy is critical to optimize dosing. Due to ethical and safety concerns the study is conducted on the pregnant baboons. A method using solid-phase extraction coupled with liquid chromatography-single quadrupole mass spectrometry for the quantitative determination of telavancin in baboon plasma samples was developed and validated. Teicoplanin was used as an internal standard. Telavancin was extracted from baboon plasma samples by using Waters Oasis(®) MAX 96-Well SPE plate and achieved extraction recovery was >66% with variation <12%. Telavancin was separated on Waters Symmetry C18 column with gradient elution. Two SIM channels were monitored at m/z 823 and m/z 586 to achieve quantification with simultaneous confirmation of telavancin identification in baboon plasma samples. The linearity was assessed in the range of 0.188μg/mL to75.0μg/mL, with a correlation coefficient of 0.998. The relative standard deviation of this method was <11% for within- and between-run assays, and the accuracy ranged between 96% and 114%.

Pharmacokinetics and safety of recently approved drugs used to treat methicillin-resistant Staphylococcus aureus infections in infants, children and adults

Methicillin-resistant Staphylococcus aureus (MRSA) remains a significant cause of morbidity in hospitalized infants. Over the past 15 years, several drugs have been approved for the treatment of S. aureus infections in adults (linezolid, quinupristin/dalfopristin, daptomycin, telavancin, tigecycline and ceftaroline). The use of the majority of these drugs has extended into the treatment of MRSA infections in infants, frequently with minimal safety or dosing information. Only linezolid is approved for use in infants, and pharmacokinetic data in infants are limited to linezolid and daptomycin. Pediatric trials are underway for ceftaroline, telavancin, and daptomycin; however, none of these studies includes infants. Here, we review current pharmacokinetic, safety and efficacy data of these drugs with a specific focus in infants.

Telavancin: a review of its use in patients with nosocomial pneumonia

Telavancin (Vibativ(®)), a lipoglycopeptide antibacterial agent, exhibits potent in vitro activity against Gram-positive bacteria associated with nosocomial pneumonia infections, including meticillin/oxacillin-resistant Staphylococcus aureus (MRSA) isolates and S. aureus isolates with reduced susceptibility to vancomycin. In two multinational trials in adult patients with nosocomial pneumonia caused by Gram-positive bacteria, including infections caused by MRSA isolates, a 1-h intravenous infusion of telavancin 10 mg/kg once daily was noninferior to intravenous vancomycin 1 g every 12 h in terms of clinical cure rates in the all-treated (AT) and clinically evaluable (CE) populations at the follow-up/test-of-cure (FU/TOC) visit. At this visit, clinical cure rates in the AT populations of both groups were approximately 60 %, with rates increasing to ≥80 % in the CE population. Pooled analyses of these trials also generally showed no significant between-group differences in clinical cure rates at the FU/TOC visit in the AT, CE and microbiologically evaluable (ME) populations. In the ME population, clinical cure rates were generally similar in the telavancin and vancomycin groups, irrespective of whether infections were mono- or polymicrobial, or caused by MRSA or methicillin/oxacillin-susceptible S. aureus isolates. Telavancin was generally well tolerated in patients with nosocomial pneumonia participating in clinical trials, with a tolerability profile that was generally similar to that of vancomycin. Telavancin offers an alternative treatment option in patients with nosocomial pneumonia caused by Gram-positive S. aureus, including those caused by MRSA and S. aureus isolates with reduced susceptibility to vancomycin.

Importance of relating efficacy measures to unbound drug concentrations for anti-infective agents

For the optimization of dosing regimens of anti-infective agents, it is imperative to have a good understanding of pharmacokinetics (PK) and pharmacodynamics (PD). Whenever possible, drug efficacy needs to be related to unbound concentrations at the site of action. For anti-infective drugs, the infection site is typically located outside plasma, and a drug must diffuse through capillary membranes to reach its target. Disease- and drug-related factors can contribute to differential tissue distribution. As a result, the assumption that the plasma concentration of drugs represents a suitable surrogate of tissue concentrations may lead to erroneous conclusions. Quantifying drug exposure in tissues represents an opportunity to relate the pharmacologically active concentrations to an observed pharmacodynamic parameter, such as the MIC. Selection of an appropriate specimen to sample and the advantages and limitations of the available sampling techniques require careful consideration. Ultimately, the goal will be to assess the appropriateness of a drug and dosing regimen for a specific pathogen and infection.

Cellular pharmacokinetic mechanisms of adriamycin resistance and its modulation by 20(S)-ginsenoside Rh2 in MCF-7/Adr cells

BACKGROUND AND PURPOSE

Intracellular pharmacokinetics of anticancer drugs in multi-drug resistance (MDR) cancer cells is hugely important in the evaluation and improvement of drug efficacy. By using adriamycin as a probe drug in MDR cancer cells, we developed a cellular pharmacokinetic-pharmacodynamic (PK-PD) model to reveal the correlation between cellular pharmacokinetic properties and drug resistance. In addition, the ability of 20(S)-ginsenoside Rh2 (20(S)-Rh2) to reverse MDR was further investigated.

EXPERIMENTAL APPROACH

The cellular pharmacokinetics of adriamycin were analysed visually and quantitatively in human breast cancer cells MCF-7 and in adriamycin-resistant MCF-7 (MCF-7/Adr) cells. Mitochondria membrane potential was assayed to evaluate the apoptotic effect of adriamycin. Subsequently, a PK-PD model was developed via MATLAB.

KEY RESULTS

Visual and quantitative data of the dynamic subcellular distribution of adriamycin revealed that it accumulated in cells, especially nuclei, to a lesser and slower extent in MCF-7/Adr than in MCF-7 cells. 20(S)-Rh2 increased the rate and amount of adriamycin entering cellular/subcellular compartments in MCF-7/Adr cells through inhibition of P-glycoprotein (P-gp) activity, in turn augmenting adriamycin-induced apoptosis. The integrated PK-PD model mathematically revealed the pharmacokinetic mechanisms of adriamycin resistance in MCF-7/Adr cells and its reversal by 20(S)-Rh2.

CONCLUSIONS AND IMPLICATIONS

P-gp, which is overexpressed and functionally active at cellular/subcellular membranes, influences the cellular pharmacokinetic and pharmacological properties of adriamycin in MCF-7/Adr cells. Inhibition of P-gp activity represents a key mechanism by which 20(S)-Rh2 attenuates adriamycin resistance. Even more importantly, our findings provide a new strategy to explore the in-depth mechanisms of MDR and evaluate the efficacy of MDR modulators.

Bone grafts as vancomycin carriers in local therapy of resistant infections

The level of an antibiotic capable of inhibiting the etiological agent at the site of infection is an essential prerequisite for successful antibiotic therapy. In some cases, locally applied antibiotics may compensate for limitations of systemic administration and shorten systemic therapy. We aimed at verifying to what extent vancomycin (Van) bound to ground bone grafts is usable in the treatment of serious infections. The levels of released Van significantly exceeded the Van minimum inhibitory concentration, which can suppress Van-sensitive staphylococci and Van intermediate Staphylococcus aureus, for the whole period of a 16-day measurement. Our results indicate that bone grafts can be used as Van carriers in therapy of osteomyelitis caused by Van-sensitive Staphylococcus strains.

Toward a new age of cellular pharmacokinetics in drug discovery

Pharmacokinetics, pharmacology, and toxicology are the major determinants of the success or failure of candidates during drug development. Because inappropriate pharmacokinetics often leads to inefficacy, even toxicity, pharmacokinetics studies have been regarded as crucial components in drug preclinical and clinical research. However, new data increasingly reveal that drug concentrations in plasma or tissues cannot totally explain the efficacy of drug on the target organ. For most drugs that interact with targets localized in cells, intracellular penetration, accumulation, distribution, and elimination are important parameters governing the efficacy in the target cells. So, there is a pressing need to clarify the cellular pharmacokinetics and thus evaluate the efficacy of drugs in the target cells. This review provides a general overview regarding current knowledge about cellular pharmacokinetics in some specific cells and also summarizes the factors that can influence cellular pharmacokinetics. It concludes by discussing potential strategies for optimizing cellular pharmacokinetics and advocating that global cellular pharmacokinetics studies be conducted in future research toward improving drug efficacy.

Cellular pharmacokinetics of the novel biaryloxazolidinone radezolid in phagocytic cells: studies with macrophages and polymorphonuclear neutrophils

Radezolid (RX-1741) is the first biaryloxazolidinone in clinical development. It shows improved activity, including against linezolid-resistant strains. Radezolid differs from linezolid by the presence of a biaryl spacer and of a heteroaryl side chain, which increases the ionization and hydrophilicity of the molecule at physiological pH and confers to it a dibasic character. The aim of this study was to determine the accumulation and subcellular distribution of radezolid in phagocytic cells and to decipher the underlying mechanisms. In THP-1 human macrophages, J774 mouse macrophages, and human polymorphonuclear neutrophils, radezolid accumulated rapidly and reversibly (half-lives of approximately 6 min and 9 min for uptake and efflux, respectively) to reach, at equilibrium, a cellular concentration 11-fold higher than the extracellular one. This process was concentration and energy independent but pH dependent (accumulation was reduced to 20 to 30% of control values for cells in medium at a pH of <6 or in the presence of monensin, which collapses pH gradients between the extracellular and intracellular compartments). The accumulation at equilibrium was not affected by efflux pump inhibitors (verapamil and gemfibrozil) and was markedly reduced at 4 degrees C but was further increased in medium with low serum content. Subcellular fractionation studies demonstrated a dual subcellular distribution for radezolid, with approximately 60% of the drug colocalizing to the cytosol and approximately 40% to the lysosomes, with no specific association with mitochondria. These observations are compatible with a mechanism of transmembrane diffusion of the free fraction and partial segregation of radezolid in lysosomes by proton trapping, as previously described for macrolides.

Activity of telavancin against Staphylococcus aureus strains with various vancomycin susceptibilities in an in vitro pharmacokinetic/pharmacodynamic model with simulated endocardial vegetations

We investigated the activity of telavancin, a novel lipoglycopeptide, alone and combined with gentamicin or rifampin (rifampicin) against strains of Staphylococcus aureus with various vancomycin susceptibilities. Strains tested included methicillin (meticillin)-resistant S. aureus (MRSA) 494, methicillin-sensitive S. aureus (MSSA) 1199, heteroresistant glycopeptide-intermediate S. aureus (hGISA) 1629, which was confirmed by a population analysis profile, and glycopeptide-intermediate S. aureus (GISA) NJ 992. Regimens of 10 mg/kg telavancin daily and 1 g vancomycin every 12 h were investigated alone and combined with 5 mg/kg gentamicin daily or 300 mg rifampin every 8 h in an in vitro model with simulated endocardial vegetations over 96 h. Telavancin demonstrated significantly greater killing than did vancomycin (P < 0.01) for all isolates except MRSA 494 (P = 0.07). Telavancin absolute reductions, in log(10) CFU/g, at 96 h were 2.8 +/- 0.5 for MRSA 494, 2.8 +/- 0.3 for MSSA 1199, 4.2 +/- 0.2 for hGISA 1629, and 4.1 +/- 0.3 for GISA NJ 992. Combinations of telavancin with gentamicin significantly enhanced killing compared to telavancin alone against all isolates (P < 0.001) except MRSA 494 (P = 0.176). This enhancement was most evident against hGISA 1629, where killing to the level of detection (2 log(10) CFU/g) was achieved at 48 h (P < 0.001). The addition of rifampin to telavancin resulted in significant (P < 0.001) enhancement of killing against only MSSA 1199. No changes in telavancin susceptibilities were observed. These results suggest that telavancin may have therapeutic potential, especially against strains with reduced susceptibility to vancomycin. Combination therapy, particularly with gentamicin, may improve bacterial killing against certain strains.

Intracellular activity of antibiotics in a model of human THP-1 macrophages infected by a Staphylococcus aureus small-colony variant strain isolated from a cystic fibrosis patient: pharmacodynamic evaluation and comparison with isogenic normal-phenotype and revertant strains

Small-colony variant (SCV) strains of Staphylococcus aureus show reduced antibiotic susceptibility and intracellular persistence, potentially explaining therapeutic failures. The activities of oxacillin, fusidic acid, clindamycin, gentamicin, rifampin, vancomycin, linezolid, quinupristin-dalfopristin, daptomycin, tigecycline, moxifloxacin, telavancin, and oritavancin have been examined in THP-1 macrophages infected by a stable thymidine-dependent SCV strain in comparison with normal-phenotype and revertant isogenic strains isolated from the same cystic fibrosis patient. The SCV strain grew slowly extracellularly and intracellularly (1- and 0.2-log CFU increase in 24 h, respectively). In confocal and electron microscopy, SCV and the normal-phenotype bacteria remain confined in acid vacuoles. All antibiotics tested, except tigecycline, caused a net reduction in bacterial counts that was both time and concentration dependent. At an extracellular concentration corresponding to the maximum concentration in human serum (total drug), oritavancin caused a 2-log CFU reduction at 24 h; rifampin, moxifloxacin, and quinupristin-dalfopristin caused a similar reduction at 72 h; and all other antibiotics had only a static effect at 24 h and a 1-log CFU reduction at 72 h. In concentration dependence experiments, response to oritavancin was bimodal (two successive plateaus of -0.4 and -3.1 log CFU); tigecycline, moxifloxacin, and rifampin showed maximal effects of -1.1 to -1.7 log CFU; and the other antibiotics produced results of -0.6 log CFU or less. Addition of thymidine restored intracellular growth of the SCV strain but did not modify the activity of antibiotics (except quinupristin-dalfopristin). All drugs (except tigecycline and oritavancin) showed higher intracellular activity against normal or revertant phenotypes than against SCV strains. The data may help rationalizing the design of further studies with intracellular SCV strains.