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Munko M, Ditzhaus M, Dobler D, Genuneit J. RMST-based multiple contrast tests in general factorial designs. Stat Med 2024; 43:1849-1866. [PMID: 38402907 DOI: 10.1002/sim.10017] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2023] [Revised: 11/13/2023] [Accepted: 01/06/2024] [Indexed: 02/27/2024]
Abstract
Several methods in survival analysis are based on the proportional hazards assumption. However, this assumption is very restrictive and often not justifiable in practice. Therefore, effect estimands that do not rely on the proportional hazards assumption are highly desirable in practical applications. One popular example for this is the restricted mean survival time (RMST). It is defined as the area under the survival curve up to a prespecified time point and, thus, summarizes the survival curve into a meaningful estimand. For two-sample comparisons based on the RMST, previous research found the inflation of the type I error of the asymptotic test for small samples and, therefore, a two-sample permutation test has already been developed. The first goal of the present paper is to further extend the permutation test for general factorial designs and general contrast hypotheses by considering a Wald-type test statistic and its asymptotic behavior. Additionally, a groupwise bootstrap approach is considered. Moreover, when a global test detects a significant difference by comparing the RMSTs of more than two groups, it is of interest which specific RMST differences cause the result. However, global tests do not provide this information. Therefore, multiple tests for the RMST are developed in a second step to infer several null hypotheses simultaneously. Hereby, the asymptotically exact dependence structure between the local test statistics is incorporated to gain more power. Finally, the small sample performance of the proposed global and multiple testing procedures is analyzed in simulations and illustrated in a real data example.
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Affiliation(s)
- Merle Munko
- Department of Mathematics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany
| | - Marc Ditzhaus
- Department of Mathematics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany
| | - Dennis Dobler
- Department of Mathematics, Faculty of Science, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
| | - Jon Genuneit
- Department of Pediatrics, Leipzig University, Leipzig, Germany
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Emura T, Ditzhaus M, Dobler D, Murotani K. Factorial survival analysis for treatment effects under dependent censoring. Stat Methods Med Res 2024; 33:61-79. [PMID: 38069825 DOI: 10.1177/09622802231215805] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/13/2024]
Abstract
Factorial analyses offer a powerful nonparametric means to detect main or interaction effects among multiple treatments. For survival outcomes, for example, from clinical trials, such techniques can be adopted for comparing reasonable quantifications of treatment effects. The key difficulty to solve in survival analysis concerns the proper handling of censoring. So far, all existing factorial analyses for survival data have been developed under the independent censoring assumption, which is too strong for many applications. As a solution, the central aim of this article is to develop new methods for factorial survival analyses under quite general dependent censoring regimes. This will be accomplished by combining existing nonparametric methods for factorial survival analyses with techniques developed for survival copula models. As a result, we will present an appealing F-test that exhibits sound performance in our simulation study. The new methods are illustrated in a real data analysis. We implement the proposed method in an R function surv.factorial(.) in the R package compound.Cox.
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Affiliation(s)
- Takeshi Emura
- Department of Statistical Data Science, The Institute of Statistical Mathematics, Tokyo, Japan
- Biostatistics Center, Kurume University, Kurume, Fukuoka, Japan
| | - Marc Ditzhaus
- Faculty of Mathematics, Otto-von-Guericke University Magdeburg, Saxony-Anhalt, Germany
| | - Dennis Dobler
- Department of Mathematics, Faculty of Science, Vrije Universiteit Amsterdam, Amsterdam, North Holland, The Netherlands
| | - Kenta Murotani
- Biostatistics Center, Kurume University, Kurume, Fukuoka, Japan
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Dobler D. Randomized empirical processes by algebraic groups, and tests for weak null hypotheses. BERNOULLI 2023. [DOI: 10.3150/22-bej1492] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/22/2023]
Affiliation(s)
- Dennis Dobler
- Department of Mathematics, Faculty of Science, Vrije Universiteit Amsterdam, The Netherlands
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Abstract
Factorial survival designs with right-censored observations are commonly inferred by Cox regression and explained by means of hazard ratios. However, in case of non-proportional hazards, their interpretation can become cumbersome; especially for clinicians. We therefore offer an alternative: median survival times are used to estimate treatment and interaction effects and null hypotheses are formulated in contrasts of their population versions. Permutation-based tests and confidence regions are proposed and shown to be asymptotically valid. Their type-1 error control and power behavior are investigated in extensive simulations, showing the new methods' wide applicability. The latter is complemented by an illustrative data analysis.
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Affiliation(s)
- Marc Ditzhaus
- Faculty of Statistics, TU Dortmund University, Dortmund, Germany
| | - Dennis Dobler
- Department of Mathematics, Vrije Universiteit Amsterdam, Amsterdam, Netherlands
| | - Markus Pauly
- Faculty of Statistics, TU Dortmund University, Dortmund, Germany
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Dobler D, Titman A. Dynamic inference for non‐Markov transition probabilities under random right censoring. Scand Stat Theory Appl 2020. [DOI: 10.1111/sjos.12443] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Dennis Dobler
- Department of Mathematics, Faculty of ScienceVrije Universiteit Amsterdam
| | - Andrew Titman
- Department of Mathematics & StatisticsLancaster University
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Feifel J, Dobler D. Dynamic inference in general nested case-control designs. Biometrics 2020; 77:175-185. [PMID: 32145031 DOI: 10.1111/biom.13259] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2019] [Revised: 02/04/2020] [Accepted: 02/25/2020] [Indexed: 11/28/2022]
Abstract
Nested case-control designs are attractive in studies with a time-to-event endpoint if the outcome is rare or if interest lies in evaluating expensive covariates. The appeal is that these designs restrict to small subsets of all patients at risk just prior to the observed event times. Only these small subsets need to be evaluated. Typically, the controls are selected at random and methods for time-simultaneous inference have been proposed in the literature. However, the martingale structure behind nested case-control designs allows for more powerful and flexible non-standard sampling designs. We exploit that structure to find simultaneous confidence bands based on wild bootstrap resampling procedures within this general class of designs. We show in a simulation study that the intended coverage probability is obtained for confidence bands for cumulative baseline hazard functions. We apply our methods to observational data about hospital-acquired infections.
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Affiliation(s)
- J Feifel
- Institute of Statistics, Ulm University, Ulm, Germany
| | - D Dobler
- Department of Mathematics, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
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Haller B, Mansmann U, Dobler D, Ulm K, Hapfelmeier A. Confidence interval estimation for the changepoint of treatment stratification in the presence of a qualitative covariate-treatment interaction. Stat Med 2020; 39:70-96. [PMID: 31701549 DOI: 10.1002/sim.8404] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2018] [Revised: 08/11/2019] [Accepted: 09/27/2019] [Indexed: 11/11/2022]
Abstract
The goal in stratified medicine is to administer the "best" treatment to a patient. Not all patients might benefit from the same treatment; the choice of best treatment can depend on certain patient characteristics. In this article, it is assumed that a time-to-event outcome is considered as a patient-relevant outcome and a qualitative interaction between a continuous covariate and treatment exists, ie, that patients with different values of one specific covariate should be treated differently. We suggest and investigate different methods for confidence interval estimation for the covariate value, where the treatment recommendation should be changed based on data collected in a randomized clinical trial. An adaptation of Fieller's theorem, the delta method, and different bootstrap approaches (normal, percentile-based, wild bootstrap) are investigated and compared in a simulation study. Extensions to multivariable problems are presented and evaluated. We observed appropriate confidence interval coverage following Fieller's theorem irrespective of sample size but at the cost of very wide or even infinite confidence intervals. The delta method and the wild bootstrap approach provided the smallest intervals but inadequate coverage for small to moderate event numbers, also depending on the location of the true changepoint. For the percentile-based bootstrap, wide intervals were observed, and it was slightly conservative regarding coverage, whereas the normal bootstrap did not provide acceptable results for many scenarios. The described methods were also applied to data from a randomized clinical trial comparing two treatments for patients with symptomatic, severe carotid artery stenosis, considering patient's age as predictive marker.
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Affiliation(s)
- Bernhard Haller
- School of Medicine, Institute for Medical Informatics, Statistics and Epidemiology, Technical University of Munich, Munich, Germany
| | - Ulrich Mansmann
- Institute for Medical Information Processing, Biometry and Epidemiology, Ludwig-Maximilians-Universität München, Munich, Germany
| | - Dennis Dobler
- Department of Mathematics, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
| | - Kurt Ulm
- School of Medicine, Institute for Medical Informatics, Statistics and Epidemiology, Technical University of Munich, Munich, Germany
| | - Alexander Hapfelmeier
- School of Medicine, Institute for Medical Informatics, Statistics and Epidemiology, Technical University of Munich, Munich, Germany
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Dobler D, Pauly M, Scheike T. Confidence bands for multiplicative hazards models: Flexible resampling approaches. Biometrics 2019; 75:906-916. [PMID: 30985914 PMCID: PMC6849815 DOI: 10.1111/biom.13059] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2018] [Revised: 03/05/2019] [Accepted: 03/18/2019] [Indexed: 11/29/2022]
Abstract
We propose new resampling‐based approaches to construct asymptotically valid time‐simultaneous confidence bands for cumulative hazard functions in multistate Cox models. In particular, we exemplify the methodology in detail for the simple Cox model with time‐dependent covariates, where the data may be subject to independent right‐censoring or left‐truncation. We use simulations to investigate their finite sample behavior. Finally, the methods are utilized to analyze two empirical examples with survival and competing risks data.
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Affiliation(s)
- Dennis Dobler
- Department of Mathematics, Vrije Universiteit Amsterdam, Amsterdam, Netherlands
| | - Markus Pauly
- Faculty of Statistics, Technical University of Dortmund, Dortmund, Germany
| | - ThomasH Scheike
- Section of Biostatistics, University of Copenhagen, Copenhagen, Denmark
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Abstract
This paper introduces new effect parameters for factorial survival designs with possibly right-censored time-to-event data. In the special case of a two-sample design, it coincides with the concordance or Wilcoxon parameter in survival analysis. More generally, the new parameters describe treatment or interaction effects and we develop estimates and tests to infer their presence. We rigorously study their asymptotic properties and additionally suggest wild bootstrapping for a consistent and distribution-free application of the inference procedures. The small sample performance is discussed based on simulation results. The practical usefulness of the developed methodology is exemplified on a data example about patients with colon cancer by conducting one- and two-factorial analyses.
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Affiliation(s)
- Dennis Dobler
- Department of Mathematics, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands
| | - Markus Pauly
- Institute of Statistics, Ulm University, Ulm, Germany
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Dobler D. Bootstrapping the Kaplan–Meier estimator on the whole line. ANN I STAT MATH 2019. [DOI: 10.1007/s10463-017-0634-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Bluhmki T, Dobler D, Beyersmann J, Pauly M. The wild bootstrap for multivariate Nelson-Aalen estimators. Lifetime Data Anal 2019; 25:97-127. [PMID: 29512005 PMCID: PMC6323102 DOI: 10.1007/s10985-018-9423-x] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/12/2017] [Accepted: 02/05/2018] [Indexed: 06/08/2023]
Abstract
We rigorously extend the widely used wild bootstrap resampling technique to the multivariate Nelson-Aalen estimator under Aalen's multiplicative intensity model. Aalen's model covers general Markovian multistate models including competing risks subject to independent left-truncation and right-censoring. This leads to various statistical applications such as asymptotically valid confidence bands or tests for equivalence and proportional hazards. This is exemplified in a data analysis examining the impact of ventilation on the duration of intensive care unit stay. The finite sample properties of the new procedures are investigated in a simulation study.
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Affiliation(s)
- Tobias Bluhmki
- Institute of Statistics, Ulm University, Helmholtzstrasse 20, 89081, Ulm, Germany
| | - Dennis Dobler
- Department of Mathematics, Vrije Universiteit Amsterdam, De Boelelaan 1081a, 1081 HV, Amsterdam, The Netherlands.
| | - Jan Beyersmann
- Institute of Statistics, Ulm University, Helmholtzstrasse 20, 89081, Ulm, Germany
| | - Markus Pauly
- Institute of Statistics, Ulm University, Helmholtzstrasse 20, 89081, Ulm, Germany
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Kiselmann C, Dobler D, Schmidts T, Eicher AC, Möbs C, Pfützner W, Runkel F. Development of a skin-friendly microemulsion for dermal allergen-specific immunotherapy. Int J Pharm 2018; 550:463-469. [PMID: 30194011 DOI: 10.1016/j.ijpharm.2018.09.002] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2018] [Revised: 08/30/2018] [Accepted: 09/04/2018] [Indexed: 01/29/2023]
Abstract
Due to their role in immune responses, the skin dendritic cells (i.e. epidermal Langerhans cells and dermal dendritic cells) have become of great interest to researchers in the past decades. A dermal administration of allergens could target these professional antigen-presenting cells directly and build up immunotolerance. Additionally, many of the adverse side effects, which are seen in the current state of the art specific immunotherapy routes, could be avoided. Therefore, in this study a penetration enhancing microemulsion was developed and its physicochemical properties were determined under several storage conditions. The influence of different preservatives on the microemulsion stability was observed. We examined epidermal penetration of Alexa Fluor-647 labelled bee-venom phospholipase A2 (Api m 1) using the Franz diffusion cell set up and confocal laser-scanning microscopy. First results of an in-vivo Api m 1-allergic mouse model indicated the protective efficacy of dermal AIT with our newly developed microemulsion. Summarily, the developed microemulsion is a suitable, stable drug delivery system for the topical administration of proteogenic allergens into the epidermis and is able to reach dendritic cells in the skin.
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Affiliation(s)
- C Kiselmann
- Institute of Bioprocess Engineering and Pharmaceutical Technology, Technische Hochschule Mittelhessen - University of Applied Sciences, Wiesenstraße 14, 35390 Giessen, Germany.
| | - D Dobler
- Institute of Bioprocess Engineering and Pharmaceutical Technology, Technische Hochschule Mittelhessen - University of Applied Sciences, Wiesenstraße 14, 35390 Giessen, Germany
| | - T Schmidts
- Institute of Bioprocess Engineering and Pharmaceutical Technology, Technische Hochschule Mittelhessen - University of Applied Sciences, Wiesenstraße 14, 35390 Giessen, Germany
| | - A C Eicher
- Institute of Bioprocess Engineering and Pharmaceutical Technology, Technische Hochschule Mittelhessen - University of Applied Sciences, Wiesenstraße 14, 35390 Giessen, Germany
| | - C Möbs
- Clinical & Experimental Allergology, Department of Dermatology and Allergology, Philipps University Marburg, Baldingerstraße, 35043 Marburg, Germany
| | - W Pfützner
- Clinical & Experimental Allergology, Department of Dermatology and Allergology, Philipps University Marburg, Baldingerstraße, 35043 Marburg, Germany
| | - F Runkel
- Institute of Bioprocess Engineering and Pharmaceutical Technology, Technische Hochschule Mittelhessen - University of Applied Sciences, Wiesenstraße 14, 35390 Giessen, Germany; Faculty of Biology and Chemistry, Justus Liebig University, Ludwigstraße 23, 35390 Giessen, Germany
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Bluhmki T, Schmoor C, Dobler D, Pauly M, Finke J, Schumacher M, Beyersmann J. A wild bootstrap approach for the Aalen-Johansen estimator. Biometrics 2018; 74:977-985. [DOI: 10.1111/biom.12861] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2018] [Revised: 12/01/2018] [Accepted: 12/01/2017] [Indexed: 11/30/2022]
Affiliation(s)
| | - Claudia Schmoor
- Clinical Trials Unit; Medical Center Freiburg; University of Freiburg; Freiburg Germany
| | - Dennis Dobler
- Institute of Statistics; Ulm University; Ulm Germany
| | - Markus Pauly
- Institute of Statistics; Ulm University; Ulm Germany
| | - Juergen Finke
- Department of Hematology; Oncology, and Stem-Cell Transplantation; Medical Center Freiburg; University of Freiburg; Freiburg Germany
| | - Martin Schumacher
- Institute for Medical Biometry and Statistics; Faculty of Medicine and Medical Center; University of Freiburg; Freiburg Germany
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Vichera G, Jordan R, Arnold V, Dobler D, Olivera R. 30 Vitrification of Equine Embryos: Application in a Commercial Cloning Program. Reprod Fertil Dev 2018. [DOI: 10.1071/rdv30n1ab30] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
During a commercial horse cloning program, a critical point is the availability and maintenance of suitable recipient mares for a large quantity of embryo transfers. Usually, pregnancy rates and viable births off the breeding season decrease significantly, whereas the rate of in vitro embryo production remains constant. For this reason, an efficient vitrification system allows continuous embryo production throughout the year with the advantage of doing the transfers only during the breeding season. The vitrification technique evaluated in this study was the one described by Kuwayama et al. (2007 Theriogenology 67, 73-80). By using this method, we compared post-warming recovery efficiency, pregnancy rates, and viable foaling rates in 2 experimental groups: cloned blastocysts vitrified off-season and transferred in breeding season (VC, n = 337), and non-vitrified cloned blastocysts also transferred in breading season (no-VC, n = 516). To achieve this, equine oocytes were collected from slaughterhouse ovaries, matured, enucleated, and fused to a donor cell according to Olivera et al. (2016 PLoS One 11, e0164049, 10.1371/journal.pone.0164049). The reconstructed embryos (RE) were cultured in a well-of-the-well system by adding 3 RE per well for 7 to 8 days to reach the blastocyst stage, at which they were vitrified as mentioned above. During the breeding season, blastocysts were warmed and transferred in couples in a single cycling receptive mare. Pregnancies were confirmed by transrectal ultrasonography 15 days post-transfer. All variables were analysed by Fisher test (P < 0.05). The warming recovery rate was 91% (308/337) for cloned blastocysts. In addition, pregnancy and viable birth rates were similar for the VC and no-VC groups: 15.6% (24/154) v. 16.7% (43/258) for pregnancy rates, respectively, and 37.5% (9/24) v. 37.2% (16/43) for foaling rates, respectively. In summary, 9 viable cloned foals were obtained with off-season embryos warmed and transferred during the breeding season, showing that vitrification did not affect embryo quality. Hence, the proposed strategy provides the ability to maximize production efficiency of equine clones by generating a large number of pregnancies without stopping in vitro embryo production at any time of the year.
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Dobler D, Pauly M. Approximate tests for the equality of two cumulative incidence functions of a competing risk. STATISTICS-ABINGDON 2017. [DOI: 10.1080/02331888.2017.1336171] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
- Dennis Dobler
- Institute of Statistics, Ulm University, Ulm, Germany
| | - Markus Pauly
- Institute of Statistics, Ulm University, Ulm, Germany
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Dobler D. A discontinuity adjustment for subdistribution function confidence bands applied to right-censored competing risks data. Electron J Stat 2017. [DOI: 10.1214/17-ejs1319] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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Schmidts T, Schlupp P, Gross A, Dobler D, Runkel F. Required HLB Determination of Some Pharmaceutical Oils in Submicron Emulsions. J DISPER SCI TECHNOL 2012. [DOI: 10.1080/01932691.2011.584800] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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Schmidts T, Dobler D, Guldan AC, Paulus N, Runkel F. Multiple W/O/W emulsions—Using the required HLB for emulsifier evaluation. Colloids Surf A Physicochem Eng Asp 2010. [DOI: 10.1016/j.colsurfa.2010.09.025] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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Schmidts T, Dobler D, Schlupp P, Nissing C, Garn H, Runkel F. Development of multiple W/O/W emulsions as dermal carrier system for oligonucleotides: effect of additives on emulsion stability. Int J Pharm 2010; 398:107-13. [PMID: 20674722 DOI: 10.1016/j.ijpharm.2010.07.037] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2010] [Revised: 07/21/2010] [Accepted: 07/21/2010] [Indexed: 11/18/2022]
Abstract
Multiple water-in-oil-in-water (W/O/W) emulsions are of major interest as potential skin delivery systems for water-soluble drugs like oligonucleotides due to their distinct encapsulation properties. However, multiple emulsions are highly sensitive in terms of variations of the individual components. The presence of osmotic active ingredients in the inner water phase is crucial for the generation of stable multiple emulsions. In order to stabilize the emulsions the influence of NaCl, MgSO(4), glucose and glycine and two cellulose derivatives was investigated. Briefly, multiple W/O/W emulsions using Span 80 as a lipophilic emulsifier and different hydrophilic emulsifiers (PEG-40/50 stearate, steareth-20 and polysorbate 80) were prepared. Stability of the emulsions was analyzed over a period of time using rheological measurements, droplet size observations and conductivity analysis. In this study we show that additives strongly influence the properties stability of multiple emulsions. By increasing the concentration of the osmotic active ingredients, smaller multiple droplets are formed and the viscosity is significantly increased. The thickening agents resulted in a slightly improved stability. The most promising emulsions were chosen and further evaluated for their suitability and compatibility to incorporate a DNAzyme oligonucleotide as active pharmaceutical ingredient.
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Affiliation(s)
- T Schmidts
- Institute of Biopharmaceutical Technology, University of Applied Sciences Giessen-Friedberg, 35390 Giessen, Germany.
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Schmidts T, Dobler D, Nissing C, Runkel F. Influence of hydrophilic surfactants on the properties of multiple W/O/W emulsions. J Colloid Interface Sci 2009; 338:184-92. [PMID: 19595359 DOI: 10.1016/j.jcis.2009.06.033] [Citation(s) in RCA: 112] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2009] [Revised: 06/04/2009] [Accepted: 06/05/2009] [Indexed: 11/30/2022]
Abstract
Multiple W/O/W emulsions for topical application using Span 80 as a lipophilic emulsifier were prepared. Several hydrophilic emulsifiers were tested in respect of their suitability for the preparation of multiple emulsions. In addition, the effect of different oil-phase compositions on emulsion stability was investigated. The physicochemical parameters of the formulations were characterized and their long-term stability was evaluated by means of rheological measurements, droplet size observations and conductivity analysis. As discovered, the modification of an oil-phase composition results in a decrease in the diffusion coefficient of water and water-soluble substances and, consequently, in enhanced stability. The influence of the release of electrolytes from the inner to the outer water phase on the emulsion stability behaviour was investigated. It was found, that the effect of the hydrophilic emulsifiers on the formulation properties is related not only to its HLB value, but rather to its chemical composition. As a result, polyethoxylated ethers of fatty alcohols (C=16-18) with HLBs between 15.3 and 16.2 appear to be the most suitable ones for creating stable formulations.
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Affiliation(s)
- T Schmidts
- Institute of Biopharmaceutical Technology, University of Applied Sciences Giessen-Friedberg, Wiesenstr. 14, 35390 Giessen, Germany.
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Oswald S, Behr G, Dobler D, Werner J, Wetzig K, Arabczyk W. Specific properties of fine SnO 2 powders connected with surface segregation. Anal Bioanal Chem 2004; 378:411-5. [PMID: 14634703 DOI: 10.1007/s00216-003-2277-3] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2003] [Revised: 08/27/2003] [Accepted: 09/05/2003] [Indexed: 10/26/2022]
Abstract
The effect of surface segregation in Sb- and In-doped SnO2 fine-grained powders has been analyzed in comparison with single-crystalline samples. The kinetics and thermodynamics of the Sb and In segregation processes were studied as a function of annealing temperature by X-ray photoelectron spectroscopy (XPS) after annealing in an oxygen-containing atmosphere. Significant differences between diffusion and segregation were revealed for doped powders and single crystals, obviously because of simultaneous diffusion and particle-growth processes proceeding during annealing of powders. For doped single crystals the thermodynamic equilibrium is approached after 24 h annealing above 850 degrees C and at 1000 degrees C for Sb and In, respectively. Higher effective activation energies of diffusion are observed for doped powders and the thermodynamic equilibrium is not achieved under technologically relevant annealing conditions. On the basis of dopant profile measurements anomalies in the electrical resistivity at 300 degrees C of Sb-doped SnO2 powders annealed at 700 and 900 degrees C were attributed to an Sb-depleted zone formed beneath the segregated surface during the kinetic regime. To achieve optimum resistivity behavior for commercial application, inhomogeneous doping of powders must be avoided by appropriate preparation steps.
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Affiliation(s)
- S Oswald
- Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, Postfach 270116, 01171 Dresden, Germany.
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Dobler D, Oswald S, Werner J, Arabczyk W, Behr G, Wetzig K. X-ray photoelectron spectroscopy investigation of segregation processes at Sb and In doped SnO2. Chem Phys 2003. [DOI: 10.1016/s0301-0104(02)00970-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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Dobler D, Oswald S, Wetzig K. Calibration of XPS - energy scale for determination of the oxidation states of doping elements in SnO 2 powders. Anal Bioanal Chem 2002; 374:646-9. [PMID: 12397485 DOI: 10.1007/s00216-002-1448-y] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2002] [Revised: 06/27/2002] [Accepted: 06/27/2002] [Indexed: 10/27/2022]
Abstract
X-ray photoelectron spectroscopy (XPS) has been used to investigate the oxidation states of doping elements in doped SnO(2) powders. Because of low conductivity, however, charging and resulting peak shift is observed. To obtain the real peak position a suitable reference peak must be found in the XPS spectrum. In this study both internal (Sn3d(5/2) peak) and external references (Au4f(7/2) and C1 s) were examined. When external references were used a shift of all the peaks studied was observed; the extent of this depended on the doping element and the doping concentration. By doping with an element of valence >4 (Nb and Sb) we obtained peaks at binding energy (BE); doping with a trivalent element (In) led to peaks at values of the BE. This peak shift is connected with changes of Fermi level. In contrast, by using Sn3d(5/2) as reference we obtained results which enabled, for example, observation of the dependence of changes of the oxidation state of Sb on doping concentration.
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Affiliation(s)
- D Dobler
- Leibniz-Institute of Solid State and Materials Research Dresden, Postfach 270016, 01171 Dresden, Germany
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