Adaptive licensing: taking the next step in the evolution of drug approval.

Traditional drug licensing approaches are based on binary decisions. At the moment of licensing, an experimental therapy is presumptively transformed into a fully vetted, safe, efficacious therapy. By contrast, adaptive licensing (AL) approaches are based on stepwise learning under conditions of acknowledged uncertainty, with iterative phases of data gathering and regulatory evaluation. This approach allows approval to align more closely with patient needs for timely access to new technologies and for data to inform medical decisions. The concept of AL embraces a range of perspectives. Some see AL as an evolutionary step, extending elements that are now in place. Others envision a transformative framework that may require legislative action before implementation. This article summarizes recent AL proposals; discusses how proposals might be translated into practice, with illustrations in different therapeutic areas; and identifies unresolved issues to inform decisions on the design and implementation of AL.

Glutathione-S-epoxide transferase in human hair follicles.

A fluorometric assay for determination of glutathione-S-epoxide transferase (GSH-T) activity in freshly isolated human hair follicles or cultured cells is described. With this assay, basal levels of the enzyme in hair follicles have been compared between smokers and non-smokers. No significant difference between both groups could be detected. GSH-T was not elevated after treatment of cultured hair follicle keratinocytes with benz[a]anthracene or clotrimazole. The same result was obtained with cultured skin fibroblasts from Ah-responsive and Ah-non-responsive mice. The possible significance of basal GSH-T levels for assessment of the risk of individuals for the carcinogenic action of polycyclic aromatic hydrocarbons is discussed.

Covalent binding of BP-metabolites to DNA of cultured human hair follicle keratinocytes.

Primary cultures of human hair follicle keratinocytes were established by using a basement membrane-like growth substrate, the bovine eye lens capsule. A method was adapted for the isolation of 3H-benzo(a)pyrene (BP)-modified DNA from the cellular outgrowth of only one hair follicle (approximately 2 X 10(5) cells). In a routine procedure hair follicle keratinocytes were incubated with 0.5 microM 3H-BP for 24 h. The purified DNA was subjected to enzymic hydrolysis and the adducts were analyzed by Sephadex LH-20 column chromatography followed by HPLC. Only one major adduct, which represented 60-80% of the total radioactivity which can be confined to modified nucleosides in the LH-20 chromatograph, could be identified. This adduct co-chromatographed with the marker adducts resulting from the trans-addition of the N-2-amino group of guanine to the 10-position of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene. Co-incubation with 7,8-benzoflavone (0.3 microM), an inhibitor of cytochrome P-448, and with 1,1,1-trichloropropene-2,3-oxide (0.2 microM), an inhibitor of epoxide hydrolase, resulted in a marked inhibitory effect (15% of the control binding) and a large increase (300% of the control value) in BP-DNA binding respectively. Induction of aryl hydrocarbon hydroxylase activity in the cultures with 5,6-benzoflavone (10 microM) or benz(a)anthracene (10 microM) caused a decrease (75 and 46% of the control value respectively) in BP-DNA binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Changing protein patterns during lens cell aging in vitro.

Changes in biosynthesis of lens proteins upon culturing have been studied by one- and two-dimensional gel electrophoretic techniques. In primary cells still growing on the capsule, alpha B2-crystallin is synthesized in a relatively high amount next to the main cytoskeletal constituents actin, tubulin and vimentin. In addition, a minor amount of beta Bp seems to be synthesized too. When the cells grow off the capsule, alpha-crystallin synthesis diminishes. beta-Crystallin synthesis continues at a low rate in cells growing on plastic or in cells forming 'lentoid bodies'. When the cells are subcultured, the synthesis of actin and vimentin becomes more pronounced, while tubulin synthesis is no longer detectable after three transfers. The relative amount of vimentin decreases, as compared to actin, during aging and elongation of the cells. When the cells have been transferred ten times and have started to elongate, a 55 kDa protein doublet differing from tubulin is observed in the two-dimensional gel patterns. We observed that elongation of lens cells in culture is accompanied by an increase in the synthesis of a polypeptide of the 26 kDa region. Furthermore, a major glycoprotein is found in the 130 kDa region, but overall glycosylation of proteins seems to decrease during lens cell elongation in vitro.

In vivo induction of aryl hydrocarbon hydroxylase in human scalp hair follicles by topical application of a commercial coal tar preparation.

Five low-dose applications of a commercial coal tar preparation on a small scalp skin region resulted in an induction of aryl hydrocarbon hydroxylase (AHH) activity in freshly isolated human hair follicles. Large but reproducible interindividual differences in AHH-inducibility could be detected. The method offers the opportunity to measure AHH-inducibility, which has been correlated to the risk of developing chemical-induced cancer, in vivo in normal epithelium, a cell-type highly relevant for chemical carcinogenesis. Smoking habits did not have any effect on AHH-activity in freshly isolated hair follicles. Therefore the method potentially permits the identification of persons with high and low genetically determined AHH-inducibility.

The use of liposomes in the topical application of steroids.

The effect of topical application of the androgen 5 alpha-dihydrotestosterone (DHT), both encapsulated in liposomes and solved in acetone, has been evaluated using the female hamster flank organ as a model system. Systemic absorption of DHT was significant from the acetone solution, but negligible from the liposome system. The topical biological effect is, however, proportionally diminished when the liposome system is used. Under the experimental conditions used, the lited using the female hamster flank organ as a model system. Systemic absorption of DHT was significant from the acetone solution, but negligible from the liposome system. The topical biological effect is, however, proportionally diminished when the liposome system is used. Under the experimental conditions used, the liposome system had no advantages over application in acetone in this model.

Human hair follicles and cultured hair follicle keratinocytes as indicators for individual differences in carcinogen metabolism.

Benzo(a)pyrene (BP)-metabolism in freshly isolated human hair follicles, cultured hair follicle keratinocytes and cells cultured from human bronchial epithelium was analysed by high performance liquid chromatography. All three types of tissues resulted in quantitatively comparable amounts of the most important organic solvent-soluble metabolites: 9,10-dihydrodiol-BP, 7,8-dihydrodiol-BP, quinones, and phenols. Besides these metabolites two early eluting compounds were detected: one possibly is BP-3-yl-hydrogen sulfate, the other probably consists of one or more tetrols. Water-soluble metabolites were quantitatively unimportant in both types of cultured cells and appeared to be primarily glucuronide and sulfate conjugates with the monohydroxides and the 7,8-dihydrodiol of BP. This metabolic pattern is compared to that of monocytes and lymphocytes which have been used frequently in population studies and with data from other types of human epithelial cells. It is concluded that human hair follicles and cultured keratinocytes from these organs are useful for detection of individual differences in carcinogen metabolism.

Formation of phenolic and dihydrodiol metabolites of benzo(a)pyrene in freshly isolated human hair follicles occurs independently.

Freshly isolated hair follicles of 20 adult non-smoking volunteers were assayed for formation of phenolic and dihydrodiol metabolites of benzo(a)pyrene (BP). In each of a total of 14 experiments two volunteers were assayed simultaneously, and the ratios of both phenolic and dihydrodiol metabolites of BP between the two individuals were determined. It was obvious that the mean interindividual variation in formation of phenolic metabolites was greater than the variation in formation of dihydrodiol metabolites. No correlation existed between the amount of both types of metabolites formed. These observations indicate that for detection of differences in carcinogen metabolism to assess individual susceptibility to the carcinogenic action of polycyclic aromatic hydrocarbons (PAH), measurements of phenolic and dihydrodiol metabolites of BP are not interchangeable.

Differences in benzo(a)pyrene metabolism between cultured human and murine bronchial cells after pre-treatment with benz(a)anthracene.

Primary cultures of human and murine (strain C3Hz) bronchial epithelial cells were pretreated with benz(a)anthracene (BA) (10 microM). 16 h later the formation of phenolic as well as dihydrodiol metabolites of benzo(a)pyrene (BP) was measured. Whereas murine cultures showed enhanced metabolism towards both phenolic and dihydrodiol compounds, in the human cultures only phenolic BP-metabolites were increased. In view of their precursor role in the formation of biologically active diol-epoxides, formation of dihydrodiol-derivatives can be considered as a key factor in determining susceptibility to polycyclic aromatic hydrocarbon (PAH)-induced carcinogenesis. Therefore the observations of this study indicate that animal model systems for PAH carcinogenesis in man have to be selected on the basis of comparable metabolite patterns.

Absence of induction of aryl hydrocarbon hydroxylase in mice after topical application of beclomethasone dipropionate.

The effects of the topical application of beclomethasone dipropionate (a corticosteroid) on the aryl hydrocarbon hydroxylase (AHH) activity in epithelial tissue (lung and skin) were studied. To evaluate the results of the pre-treatment with beclomethasone dipropionate a comparison was made with the effects of the topical application of benzanthracene (a polycyclic aromatic hydrocarbon and a known inducer of AHH activity). AHH activities were increased 2.5 to 5-fold in lung and about 20-fold in skin of mice after topical application of benzanthracene. However, after pre-treatment with beclomethasone dipropionate no induction of AHH activity could be observed, either in lung or in skin. The implications of these findings are discussed.

Formation of dihydrodiol metabolites of benzo(alpha)pyrene in cultured human and murine skin cells.

The formation of the 7, 8-a and 9, 10-dihydrodiol metabolites of benzo(a)pyrene, which are believed to play a role in the chemical induction of tumors, was investigated in cultures of human and murine origin. It was found that cultures of mouse (strain C3Hz) epidermal and skin fibroblastic cells showed inducible benzo(a)pyrene metabolism towards dihydrodiol metabolites, after pre-incubation with benz(a)anthracene. This was consistent with the increased in vivo formation of dihydrodiol metabolites of benzo(a)pyrene after injection with 3-methylcholanthrene. In contrast, in human cell cultures the metabolism of benzo(a)pyrene to the dihydrodiol metabolites was not enhanced after pre-exposure to benz(a)anthracene. This was the case in low-passage skin fibroblasts, primary epidermal skin cells, and primary keratinocytes from hair follicles. Moreover, other inducers of microsomal oxygenases, such as phenobarbital and 3-methylcholanthrene, were also unable to increase benzo(a)pyrene metabolism towards the dihydrodiol compounds. In view of these results, obtained using in vitro human and murine model systems, we may conclude that human and murine skin cell culture systems respond differently to pre-treatment with inducers of microsomal monooxygenases with respect to the metabolism of benzo(a)pyrene to reactive dihydrodiol metabolites. The possible implications for the human in vivo situation are discussed.

Human hair follicles, a convenient tissue for genetic studies on carcinogen metabolism.

Basal levels of benzo(a)pyrene metabolism were measured in hair follicles of seven monozygotic twins, eight dizygotic twins and ten pairs of unrelated individuals. Organic soluble metabolites were separated by thin-layer chromatography, visualised by autoradiography and quantified by scanning of the films. Activity was expressed as pmol 7,8- and 9,10-dihydrodiol metabolites of benzo(a)pyrene per micrograms DNA per hour. Intra-twin differences in benzo(a)pyrene metabolism for monozygotic twins were smaller than for dizygotic twins and intra-pair differences for dizygotic twins were smaller for pairs of unrelated individuals. The results clearly suggest that individual differences in benzo(a)pyrene metabolism in hair follicles are partly genetically determined. Thus, hair follicles may be used for investigations on the suggested relations between genetic predisposition to carcinogen-induced cancer and individual differences in carcinogen metabolism. The relevance of these findings to research into the induction of neoplasms by carcinogens in epithelial tissues is discussed.

Enzyme induction in cultured human hair follicle cells.

The enzyme aryl hydrocarbon hydroxylase is present in murine and human cultures of keratinocytes. While the activity of aryl hydrocarbon hydroxylase in murine cultures was found to be inducible after exposure to benz(a)anthracene, human keratinocytes originating from the hair follicle did not respond to this treatment. Hence, cultured human keratinocytes are not suitable for studying the molecular events that mediate the process of aryl hydrocarbon hydroxylase induction.

A simple fluorimetric microassay for DNA in hair follicles or fractions of hair follicles.

Human hair follicles may be useful for determining individual differences in the metabolism of polycyclic aromatic hydrocarbons in epithelial tissues. for quantitative measurements of carcinogen metabolism, determination of the amount of DNA in the hair follicles is necessary in order to correct for individual size variation. A simple method for DNa determination in hair follicles is described, based on the use of a hypotonic pronase solution to solubilise DNA which is then available for complex formation with mithramycin or 4,6-diamidino-2-phenylindole. 2HCI (DAPI). The mithramycin method permits the measurement of DNA in a small number of hair follicles, while the DAPI method is sensitive for as little as one single bulb.