Hepatitis C virus resistance to new specifically-targeted antiviral therapy: A public health perspective

Development of latent Interferon alpha 2b as a safe therapeutic for treatment of Hepatitis C virus infection

Interferon therapy for the treatment of hepatitis C virus infection has very limited clinical application due to short serum half-life and side effects of therapy in systemic route of administration. In the present study, we have focused to improve the interferon therapy by overcoming the limitation of side effects. We hypothesized that latent interferon alpha 2b (IFNα2b) produced by fusion of Latency associated protein (LAP) domain of TGFβ and IFNα2b having HCV NS3 protease cleavage site as linker that will be activated only at target site (liver) by viral protease (HCV NS3 protease) present on the surface of infected cells. The fusion proteins were expressed in pichia pastoris as homodimer and cleaved by recombinant HCV NS3 protease in vitro into two fragments corresponding to the IFNα-2b and LAP respectively. The latency of chimeric proteins and biological activity after treatment with HCV NS3 protease was assessed by cytopathic effect inhibition assay in A594 cells infected with encephalomyocarditis virus (EMCV) and reduction in HCV viral load in Huh7 cells. The HCV NS3 protease was present on the surface of HCV replicating Huh7 cells in amount that activated half of the effective concentration (EC₅₀) of latent IFNα2b fusion protein. As free circulating HCV NS3 protease was not detected in sera from chronic HCV patients and in vitro cleavage of intact latent IFNα2b fusion protein was not observed with peripheral blood mononuclear cells (PBMCs) isolated from chronic HCV patients, thus there are less likely chances of activation and off target binding of latent IFNα2b to show side effects during systemic route of administration. Therefore, most of the side effects of interferon can be overwhelmed at the cost of 50% reduced biological activity. Thus, the use of latent IFNα2b can be considered again as an option for treatment of HCV infection in combination with direct acting antivirals rather than alone with improved safety profile.

Identification of drug resistance and immune-driven variations in hepatitis C virus (HCV) NS3/4A, NS5A and NS5B regions reveals a new approach toward personalized medicine

Cellular immune responses (T cell responses) during hepatitis C virus (HCV) infection are significant factors for determining the outcome of infection. HCV adapts to host immune responses by inducing mutations in its genome at specific sites that are important for HLA processing/presentation. Moreover, HCV also adapts to resist potential drugs that are used to restrict its replication, such as direct-acting antivirals (DAAs). Although DAAs have significantly reduced disease burden, resistance to these drugs is still a challenge for the treatment of HCV infection. Recently, drug resistance mutations (DRMs) observed in HCV proteins (NS3/4A, NS5A and NS5B) have heightened concern that the emergence of drug resistance may compromise the effectiveness of DAAs. Therefore, the NS3/4A, NS5A and NS5B drug resistance variations were investigated in this study, and their prevalence was examined in a large number of protein sequences from all HCV genotypes. Furthermore, potential CD4+ and CD8+ T cell epitopes were predicted and their overlap with genetic variations was explored. The findings revealed that many reported DRMs within NS3/4A, NS5A and NS5B are not drug-induced; rather, they are already present in HCV strains, as they were also detected in HCV-naïve patients. This study highlights several hot spots in which HLA and drug selective pressure overlap. Interestingly, these overlapping mutations were frequently observed among many HCV genotypes. This study implicates that knowledge of the host HLA type and HCV subtype/genotype can provide important information in defining personalized therapy.

Prevalence and characteristics of naturally occurring sofosbuvir resistance-associated variants in patients with hepatitis C virus genotype 1b infection

AIM

Sofosbuvir (SOF), a nucleotide analog pro-drug, targets hepatitis C virus (HCV) NS5B polymerase and shows potential for treating HCV infection, given its high efficacy and good barrier to resistance. However, in addition to the rare resistant-associated variant (RAV) of non-structural protein NS5B S282T, several new potential RAVs of SOF have been reported, especially related to HCV genotype 1b. However, the prevalence and characteristics of these RAVs have not been clarified.

METHODS

We analyzed the prevalence of variants in the NS3/NS5A/NS5B regions in 96 patients treated with simeprevir (SMV) combination therapy, and the prevalence of RAVs in patients showing treatment failure was determined by direct- or deep-sequencing methods. Associations between these potential RAVs and clinical factors were also analyzed.

RESULTS

Prevalence of NS5B RAV C316N was high (46.9%, 45/96), whereas that of NS5B L159F was relatively low (1.04%, 1/96); however, deep sequencing showed that 30.0% of patients with C316N also had NS5B RAV L159F. Additionally, there was no significant relationship between the existence of potential NS5B and NS5A or NS3 RAVs. However, the presence of NS5B C316N was significantly associated with an HCV core amino acid 91 substitution. No significant difference was detected between each RAV and sustained virological response in simeprevir combination therapy.

CONCLUSION

We provide clear evidence of the high prevalence of two potential naturally occurring NS5B RAVs (C316N and L159F) in Japan. It may be important to pay particular attention to these new potential RAVs, especially when using SOF-based therapy in patients with RAVs due to previous direct-acting antiviral therapy failure.

Resistance-associated mutations to HCV protease inhibitors naturally pre-existed in HIV/HCV coinfected, treatment-naïve patients

BACKGROUND AND OBJECTIVE

Presently, it has been approved to treat hepatitis C virus (HCV) infection with protease inhibitors (PIs). In the present study, we aimed to illustrate the natural prevalence of HCV NS3/4A PI resistance mutations in HIV/HCV coinfected, treatment-naïve patients.

METHODS

Population sequence analysis of the NS3/4A protease was conducted in 96 HIV/HCV coinfected, treatment-naïve patients.

RESULTS

Complete HCV NS3/4A sequence information was obtained from 70 (72.92%) samples, including 68 patients (97.14%) with genotype 1b and 2 patients (2.86%) with genotype 2a. A total of 21 patients (30.88%) of the 68 patients with HCV genotype 1b showed amino acid substitutions associated with HCV PI resistance. Mutation F43S was observed in 1.47% of patients with genotype 1b. The mutations of T54S, Q80K/R, R155K, A156G and D168A/E/G were found in 4.41%, 1.47%/1.47%, 2.94%, 23.53% and 1.47%/1.47%/1.47% of patients with genotype 1b, respectively. In addition, 4.41% of patients with genotype 1b showed double mutations in the NS3 region. The multiple mutations of Q80R+R155K+D168G and Q80K+R155K+A156G+D168A were detected in 1.47% and 1.47% of patients with HCV genotype 1b, respectively.

CONCLUSIONS

The most predominant HCV genotype was 1b in patients with HIV/HCV coinfection. Naturally occurring mutations resistant to HCV PIs (simeprevir, vaniprevir, boceprevir, telaprevir, asunaprevir and paritaprevir) pre-existed in patients with HIV/HCV genotype 1b coinfection. The effects of baseline PI resistance on treatment outcome should be further analyzed.

Biomedical Mutation Analysis (BMA): A software tool for analyzing mutations associated with antiviral resistance

INTRODUCTION

Hepatitis C virus (HCV) is considered a major public health problem, with 200 million people infected worldwide. The treatment for HCV chronic infection with pegylated interferon alpha plus ribavirin inhibitors is unspecific; consequently, the treatment is effective in only 50% of patients infected. This has prompted the development of direct-acting antivirals (DAA) that target virus proteins. These DAA have demonstrated a potent effect in vitro and in vivo; however, virus mutations associated with the development of resistance have been described.

OBJECTIVE

To design and develop an online information system for detecting mutations in amino acids known to be implicated in resistance to DAA.

MATERIALS AND METHODS

   We have used computer applications, technological tools, standard languages, infrastructure systems and algorithms, to analyze positions associated with resistance to DAA for the NS3, NS5A, and NS5B genes of HCV.

RESULTS

We have designed and developed an online information system named Biomedical Mutation Analysis (BMA), which allows users to calculate changes in nucleotide and amino acid sequences for each selected sequence from conventional Sanger and cloning sequencing using a graphical interface.

CONCLUSION

BMA quickly, easily and effectively analyzes mutations, including complete documentation and examples. Furthermore, the development of different visualization techniques allows proper interpretation and understanding of the results. The data obtained using BMA will be useful for the assessment and surveillance of HCV resistance to new antivirals, and for the treatment regimens by selecting those DAA to which the virus is not resistant, avoiding unnecessary treatment failures. The software is available at: http://bma.itiud.org.

Biomedical Mutation Analysis (BMA): A software tool for analyzing mutations associated with antiviral resistance

INTRODUCTION

Hepatitis C virus (HCV) is considered a major public health problem, with 200 million people infected worldwide. The treatment for HCV chronic infection with pegylated interferon alpha plus ribavirin inhibitors is unspecific; consequently, the treatment is effective in only 50% of patients infected. This has prompted the development of direct-acting antivirals (DAA) that target virus proteins. These DAA have demonstrated a potent effect in vitro and in vivo; however, virus mutations associated with the development of resistance have been described.

OBJECTIVE

To design and develop an online information system for detecting mutations in amino acids known to be implicated in resistance to DAA.

MATERIALS AND METHODS

   We have used computer applications, technological tools, standard languages, infrastructure systems and algorithms, to analyze positions associated with resistance to DAA for the NS3, NS5A, and NS5B genes of HCV.

RESULTS

We have designed and developed an online information system named Biomedical Mutation Analysis (BMA), which allows users to calculate changes in nucleotide and amino acid sequences for each selected sequence from conventional Sanger and cloning sequencing using a graphical interface.

CONCLUSION

BMA quickly, easily and effectively analyzes mutations, including complete documentation and examples. Furthermore, the development of different visualization techniques allows proper interpretation and understanding of the results. The data obtained using BMA will be useful for the assessment and surveillance of HCV resistance to new antivirals, and for the treatment regimens by selecting those DAA to which the virus is not resistant, avoiding unnecessary treatment failures. The software is available at: http://bma.itiud.org.

Hepatitis C virus-cross-reactive TCR gene-modified T cells: a model for immunotherapy against diseases with genomic instability

A major obstacle hindering the development of effective immunity against viral infections, their associated disease, and certain cancers is their inherent genomic instability. Accumulation of mutations can alter processing and presentation of antigens recognized by antibodies and T cells that can lead to immune escape variants. Use of an agent that can intrinsically combat rapidly mutating viral or cancer-associated antigens would be quite advantageous in developing effective immunity against such disease. We propose that T cells harboring cross-reactive TCRs could serve as a therapeutic agent in these instances. With the use of hepatitis C virus, known for its genomic instability as a model for mutated antigen recognition, we demonstrate cross-reactivity against immunogenic and mutagenic nonstructural protein 3:1406-1415 and nonstructural protein 3:1073-1081 epitopes in PBL-derived, TCR-gene-modified T cells. These single TCR-engineered T cells can CD8-independently recognize naturally occurring and epidemiologically relevant mutant variants. TCR-peptide MHC modeling data allow us to rationalize how TCR structural properties accommodate recognition of certain mutated epitopes and how these substitutions impact the requirement of CD8 affinity enhancement for recognition. A better understanding of such TCRs' promiscuous behavior may allow for exploitation of these properties to develop novel, adoptive T cell-based therapies for viral infections and cancers exhibiting similar genomic instability.

Assessment of the activity of directly acting antivirals and other products against different genotypes of hepatitis C virus prevalent in resource-poor countries

Certain food additives and drugs used for other indications have been shown to inhibit in vitro replication of HCV and have been proposed as cheap options for the treatment of HCV infections in resource-poor countries. We here report that the in vitro anti-HCV (genotypes 1a, 1b, 2a and 4b) activity of nitazoxanide, silymarin, silibinin and the green tea extract EGCG is very weak when compared to directly acting antivirals. HCV-infected patients in resource-poor countries should receive the best possible treatment (if possible via expanded access programs); it is therefore advisable not to plan clinical studies with drugs/compounds with weak anti-HCV activity.

Naturally occurring, resistance-associated hepatitis C virus NS5A variants are linked to interleukin-28B genotype and are sensitive to interferon-based therapy

AIM

The presence of resistance-associated variants (RAV) may attenuate the efficacy of direct-acting antivirals (DAA) in combination therapy for hepatitis C. The aim of this study was to characterize the NS3 and NS5A regions of hepatitis C virus (HCV) in naturally occurring RAV.

METHODS

The NS3 and NS5A regions of HCV were amplified by nested polymerase chain reaction and their nucleotide sequences were determined by direct sequencing in 493 genotype 1b patients naive to DAA-based therapies. The effect of baseline RAV on response to pegylated interferon and ribavirin therapy was analyzed in 65 patients after stratification by interleukin (IL)-28B genotype.

RESULTS

The incidence of RAV was 7.9% in NS3 (V36I/L, 1.2%; T54S, 2.8%; Q80K/R, 3.0%; A156S, 0.2%; and D168E/T, 2.4%) and 20.2% in NS5A (L31I/M, 2.2%; and Y93H, 19.0%). The incidence in interferon experienced and naive patients was similar. The incidence of Y93H in NS5A was significantly higher in the IL-28B TT genotype (rs8099917) than non-TT (27.1% vs 9.5%, P < 0.001). The virological response to pegylated interferon plus ribavirin therapy was not affected by the presence of RAV in IL-28B TT genotype.

CONCLUSION

RAV, especially Y93H in the NS5A region, were highly prevalent in DAA naive patients with genotype 1b HCV in Japan and were linked to IL-28B TT genotype. Interferon-based therapy could be an alternative for patients with RAV because these variants did not attenuate the response to that therapy. The analysis of RAV may impact the selection of the optimal treatment strategy.

HCV NS3 quasispecies in liver and plasma and dynamics of telaprevir-resistant variants in breakthrough patients assessed by UDPS: A case study

BACKGROUND

The impact of pre-existing variants in hepatitis C virus (HCV) quasispecies, carrying resistance-associated mutations (RAMs), on the outcome of treatment with direct acting antiviral agents (DAA) is debated and it is complicated by the lack of knowledge of quasispecies distribution between the viral reservoir (liver) and the circulating compartment.

OBJECTIVE

To evaluate NS3 protease heterogeneity and presence of RAMs on baseline plasma and liver biopsy samples. Plasma dynamics were also analyzed during therapy and after its suspension. Study design Ultra-deep pyrosequencing (UDPS) was performed in two HCV genotype 1a patients who received telaprevir (TVR)-based therapy and developed treatment failure due to TVR-resistance.

RESULTS

In both patients the baseline diversity of NS3 quasispecies in plasma was higher than in liver (183.6×10(-4) vs 47.8×10(-4) and 246.0×10(-4) vs 55.0×10(-4) nt substitution/site, respectively, p<0.0001), but phylogenetic trees did not evidence compartmentalization between the two compartments. At baseline RAMs (i.e. V36A, T54A) were detected very low levels (range: 0.31-0.52%) in both specimen types. However, phylogenetic analyses revealed that the viral variants carrying these mutations at baseline were different from those that became fixed at breakthrough, when combined V36M+R155K, conferring high-level resistance to TVR, were observed. The frequency of resistance-associated variants declined after withdrawal of drug selective pressure.

CONCLUSIONS

UDPS allowed extensive evaluation of quasispecies compartmentalization and of their dynamics after withdrawal of TVR. Plasma and liver NS3 quasispecies, including low level RAMs, do not show significant difference.

Management of telaprevir-based triple therapy for hepatitis C virus recurrence post liver transplant

AIM: To characterize management of telaprevir (TVR)-based triple therapy of hepatitis C virus (HCV) reinfection after liver transplantation (LT).

METHODS: We retrospectively analyzed safety and efficacy of telaprevir - based triple therapy in a single center cohort of 19 patients with HCV genotype (GT) 1 recurrence after LT, with respect to factors possibly predicting sustained viral response (SVR) or non-SVR. All patients were treated with TVR, pegylated (PEG) and ribavirine (RBV) for 12 wk followed by a dual phase with PEG/RBV for 12 wk in 7 patients and for 36 wk in 5 patients.

RESULTS: In total 11/19 (58%) of patients achieved a sustained response. All (11/11) SVR patients showed a rapid viral response at treatment weeks 4 and 11/14 rapid virological response (RVR) patients achieved SVR. Notably, all (7/7) patients who completed 48 wk of therapy and 80% (4/5) patients who completed 24 wk of therapy achieved SVR24. Treatment failure was significantly (P > 0.049) more frequent in GT1a infection (5/7) compared to GT1b (3/12) infection and was associated with emergence of resistance-associated mutations in the NS3 protease domain. Bilirubin level at baseline is also related to SVR (P > 0.030). None of the patients had to discontinue treatment due to side effects.

CONCLUSION: RVR, GT and bilirubin are clearly related to achievement of SVR. Providing a thorough patient selection and monitoring, a full course of TVR-based triple therapy in LT patients is feasible and achieves high SVR rates.

Finding Truth in a World Full of Spin: Myth-Busting in the Case of Sovaldi

PURPOSE

Public discourse regarding the hepatitis C virus (HCV) drug Sovaldi® (sofosbuvir) has become inflamed, generating much heat but little light concerning the clinical, health economic, and quality-of-life merits of Sovaldi®. The purpose of this article is to provide a factual basis for evaluating the claims regarding the benefits of Sovaldi® relative to its costs.

METHODS

A comprehensive review was conducted of news stories highlighted in the daily updates of the electronic newsletters BIO SmartBrief, FiercePharma, FierceBiotech and BioCentury Extra published from November 1, 2013, through December 31, 2014, on the topics of the HCV market, Sovaldi®, and other HCV therapeutics. Also reviewed were recent practice guidelines on the management of HCV infections, prescribing information on all HCV drugs approved by the US Food and Drug Administration, and health technology assessments of Sovaldi® and Harvoni(TM) (sofosbuvir/ledipasvir).

FINDINGS

Sovaldi® and Harvoni(TM) have provided significant improvements in the treatment of HCV, with all-oral regimens and cure rates exceeding 90% in some populations of patients with HCV. Sovaldi® prevents significant health care resource utilization in patients who would otherwise develop cirrhosis and require a liver transplant; however, only a small proportion of patients with HCV develop cirrhosis, and fewer require liver transplants. Because it is not possible to identify those patients whose HCV will progress to severe liver disease, it would be necessary to treat a large number of patients with HCV to prevent disease progression in this subpopulation, resulting in a considerable loss to health plans even over a 20-year horizon. The claim that treating all patients with HCV with Sovaldi® would cost nearly as much as the current total US expenditure on all prescription drugs, while factually correct, is not a realistic scenario. Many patients with HCV will continue to go undiagnosed. In addition, the medical expense for those who are treated will be spread out over many years. However, the unexpectedly large, up-front cost of covering these drugs has had a major impact on health plan budgets, resulting in losses for some plans.

IMPLICATIONS

Sovaldi® represents an enormous advance in the care of some populations of HCV-infected patients, but also a major cost burden to health plans. As the first of a number of anticipated, paradigm-changing drugs to treat medical conditions affecting large patient populations, Sovaldi® should act as a wake-up call for all health care stakeholders to engage in a meaningful, fact-based discussion about managing the cost of innovative new drugs to balance the needs of drug manufacturers, health plans, providers, and, above all, patients.

Advanced molecular surveillance of hepatitis C virus

Hepatitis C virus (HCV) infection is an important public health problem worldwide. HCV exploits complex molecular mechanisms, which result in a high degree of intrahost genetic heterogeneity. This high degree of variability represents a challenge for the accurate establishment of genetic relatedness between cases and complicates the identification of sources of infection. Tracking HCV infections is crucial for the elucidation of routes of transmission in a variety of settings. Therefore, implementation of HCV advanced molecular surveillance (AMS) is essential for disease control. Accounting for virulence is also important for HCV AMS and both viral and host factors contribute to the disease outcome. Therefore, HCV AMS requires the incorporation of host factors as an integral component of the algorithms used to monitor disease occurrence. Importantly, implementation of comprehensive global databases and data mining are also needed for the proper study of the mechanisms responsible for HCV transmission. Here, we review molecular aspects associated with HCV transmission, as well as the most recent technological advances used for virus and host characterization. Additionally, the cornerstone discoveries that have defined the pathway for viral characterization are presented and the importance of implementing advanced HCV molecular surveillance is highlighted.

MICA SNPs and the NKG2D system in virus-induced HCC

Hepatocellular carcinoma (HCC) is one of the most frequent causes of cancer-related death globally. Above well-known risk factors for HCC development ranging from various toxins to diseases such as diabetes mellitus, chronic infection with hepatitis B virus and hepatitis C virus (HCV) poses the most serious threat, constituting the cause in more than 80 % of cases. In addition to the viral genes intensively investigated, the pathophysiological importance of host genetic factors has also been greatly and increasingly appreciated. Genome-wide association studies (GWAS) comprehensively search the host genome at the single-nucleotide level, and have successfully identified the genomic region associated with a whole variety of diseases. With respect to HCC, there have been reports from several groups on single nucleotide polymorphisms (SNPs) associated with hepatocarcinogenesis, among which was our GWAS discovering MHC class I polypeptide-related sequence A (MICA) as a susceptibility gene for HCV-induced HCC. MICA is a natural killer (NK) group 2D (NKG2D) ligand, whose interaction with NKG2D triggers NK cell-mediated cytotoxicity toward the target cells, and is a key molecule in tumor immune surveillance as its expression is induced on stressed cells such as transformed tumor cells for the detection by NK cells. In this review, the latest understanding of the MICA-NKG2D system in viral HCC, particularly focused on its antitumor properties and the involvement of MICA SNPs, is summarized, followed by a discussion of targets for state-of-the-art cancer immunotherapy with personalized medicine in view.

Present, old and future strategies for anti-HCV treatment in patients infected by genotype-1: estimation of the drug costs in the Calabria Region in the era of the directly acting antivirals

BACKGROUND

In Italy, anti-HCV drugs are provided free of charge by the National Health System. Since 2011, three drug regimens including a directly acting antiviral (DAA) are considered the gold standard for HCV treatment. However, these drugs add a significant cost (roughly €26,000) to the combination of pegylated-interferon-α/ribavirin (PEG-IFN/RBV), which before DAA represented the unique treatment. To provide the National Health System potential useful information, we estimated costs to provide anti-HCV drugs to treat a population experienced for PEG-INF/RBV.

METHODS

Genotype 1 HCV mono-infected or HIV/HCV co-infected individuals who were treated with PEG-IFN/RBV between 2008 and 2013 were included. The cost to treat these patients with PEG-IFN/RBV was calculated (cost 1). We also estimated costs if we had to treat these patients with a lead-in period of PEG-INF/RBV followed by PEG-IFN/RBV and a DAA in naïves (cost 2), in addition to cost 1 plus the estimated cost to re-treat with PEG-IFN/RBV and a DAA patients who had a relapse or a non response (cost 3). Moreover, all costs were normalized by SVR. Rates of foreseen response with DAA were obtained from literature data.

RESULTS

The overall study population consisted of 104 patients. The rate of sustained virological response (SVR) was 55%, while it was estimated that SVR would be obtained in 75% of patients with a lead-in period with PEG-IFN/RBV followed by a DAA combination, and in 78% if this treatment is used to re-treat experienced patients with a DAA. Drug costs associated with these treatments were: €1,214,283 for cost 1, €3,474,977 for cost 2 and €3,002,095 for cost 3. Costs per SVR achieved were: €22,284 for cost 1, €44,643 for cost 2 and €38,322 for cost 3.

CONCLUSIONS

Treatments including DAAs achieve a SVR in more patients than PEG-IFN/RBV but they cost around three times more than PEG-IFN/RBV alone regimens. Also, cost per SVR is almost twofold greater than PEG-IFN/RBV regimens. Therefore, it is mandatory to implement use of DAA in clinical practice, but the National Health System should allocate adequate resources to provide drugs, which challenges sustainability. Cost reduction for anti-HCV drugs should be pursued.

Aryl-substituted aminobenzimidazoles targeting the hepatitis C virus internal ribosome entry site

We describe the exploration of N1-aryl-substituted benzimidazoles as ligands for the hepatitis C virus (HCV) internal ribosome entry site (IRES) RNA. The design of the compounds was guided by the co-crystal structure of a benzimidazole viral translation inhibitor in complex with the RNA target. Structure-binding activity relationships of aryl-substituted benzimidazole ligands were established that were consistent with the crystal structure of the translation inhibitor complex.

Calyculin A, an enhancer of myosin, speeds up anaphase chromosome movement

Actin and myosin inhibitors often blocked anaphase movements in insect spermatocytes in previous experiments. Here we treat cells with an enhancer of myosin, Calyculin A, which inhibits myosin-light-chain phosphatase from dephosphorylating myosin; myosin thus is hyperactivated. Calyculin A causes anaphase crane-fly spermatocyte chromosomes to accelerate poleward; after they reach the poles they often move back toward the equator. When added during metaphase, chromosomes at anaphase move faster than normal. Calyculin A causes prometaphase chromosomes to move rapidly up and back along the spindle axis, and to rotate. Immunofluorescence staining with an antibody against phosphorylated myosin regulatory light chain (p-squash) indicated increased phosphorylation of cleavage furrow myosin compared to control cells, indicating that calyculin A indeed increased myosin phosphorylation. To test whether the Calyculin A effects are due to myosin phosphatase or to type 2 phosphatases, we treated cells with okadaic acid, which inhibits protein phosphatase 2A at concentrations similar to Calyculin A but requires much higher concentrations to inhibit myosin phosphatase. Okadaic acid had no effect on chromosome movement. Backward movements did not require myosin or actin since they were not affected by 2,3-butanedione monoxime or LatruculinB. Calyculin A affects the distribution and organization of spindle microtubules, spindle actin, cortical actin and putative spindle matrix proteins skeletor and titin, as visualized using immunofluorescence. We discuss how accelerated and backwards movements might arise.