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Taxol: The chemistry and structure-activity relationships of a novel anticancer agent
222
Taxol: the chemistry and structure-activity relationships of a novel anticancer agent David G. I. Kingston Taxol is an exciting new anticancer drug, showing clinical activity against ovarian and breast cancer. Its development as a clinically useful drug has involved major efforts to overcome the supply problem; this has now been done, and the focus of interest has moved to the development of improved analogs of the drug. Recent notable achievements include the first total synthesis of taxol, and the first indications of its binding site on tubulin. For many centuries, the yew tree was regarded as the tree of death, and its toxic constituents provided a death potion in Julius Caesar's time. It is thus both ironic and encouraging that tiffs same tree should now give cause for hope in the fight against cancer. The diterpenoid taxol (Structure 1) was isolated
AcO IB"4 1 C6HsCON H
O
T ~"
sO
2~,~/__/~-~9 ~'~
OH
I
3[
6 ],5
)i. OH
~COCsH 5
1 from the Western Yew (Taxus brevifolia) by Monroe Wall and his collaborators in the late 1960s, and its structure was published in 19711. Although taxol showed good activity in the cytotoxicity assays and in vivo antileukemic assays then in use, it was not given a high priority for development because of two obvious obstacles to its success as an anticancer drug: (1) its very low solubility in water, making it difficult to formMate; and (2) its very low yield from the bark o f T. brevifolia. Fortunately, additional biological testing did continue, and in the mid-1970s, taxol was found to show activity in some new bioassays, including the B 16 melanoma assay and three human-tumorxenograft assays in nude mice 2. Based on these new data, taxol was advanced into preclinical development by the US National Cancer Institute (NCI) in 1977. D. G. L Kingston b at the Department of Chemistry, Virginia Polytechnic Institute and State University, BIacksburg, VA 24061-0212, USA. TIBTECHJUNE1994(VOL12)
An important discovery in •979 reinforced the earlier decision to investigate taxol further, and generated significant interest in taxol in the biological community. In that year, Susan Horwitz and her collaborators made the important discovery that taxol promotes polymerization o f the cellular protein tubulin, causing it to assemble into stable microtubules 3. It is believed that this mechanism is responsible for taxol's action as an antimitotic drug, and taxol has thus become a valuable biochemical tool for studying mitosis. Taxol entered Phase I clinical trials in 1983, but immediately ran into some problems related to its formulation. As noted earlier, it is very insoluble in water, and it was eventually formulated as an emulsion with Cremophor EL ®, a polyethoxylated castor oil. As taxol must be given at relatively high dosages, large amounts of Cremophor EL ® were required; this caused problems with allergic reactions in some patients, including one death 4. Fortunately, these problems were overcome by lengthening the infusion period and premedicating patients with glucocorticoids and antihistamines. Phase II trials were initiated in 1985 and these proved to be very successful: taxol was found to have excellent activity against drug-refractory ovarian cancer5 and against breast cancer 6. It was approved by the US Food and Drug Administration (FDA) for the treatment o f ovarian cancer in 1992.
Chemistry and structure-activity relationships of taxol Extensive chemical characterization o f taxol has been carried out (see Ret~ 7, 8 for review). These studies have provided a still-incomplete but, nevertheless, substantial body of information on structure-activity relationships, which is summarized below and in Fig. 1. © 1994, ElsevierScienceLtd
223
fOCUS
Acetyl or acetoxy group may be removed without significant loss of activity C
Reduction improves activity slightly /
~ ¢/ O 19 OH
AcO N-awl group. ~ O required /~~.,,~
18 -NH
1
~
~12./~i
~
II
]
--
.OH
~
~--';~-7, -vnenylgroup or a close analog required
6 81
~-
I
~
"~.~"~'L~I 9 ~ ! ( x ~=~N./" O la (~w - Acu ~
~1
~ //"
May be esterified, epimerized or removedwithout significantloss of activity
4 C~".O
~ t -Free 2'-hydroxyl group, or a hydrolysable ester thereof required
IO~e~an$.rlng I required fo I activity
="
~~ # ( ~
~
'
~
i[ Removalof acetate ] [ reduces activity slightly
-!
Benzoyloxygroup essential; certain substituted groups have improved activity
Figure 1
Structure-activityrelationshipsoftaxol. The side chain The N-benzoyl-13-phenylisoserine side chain of taxol can be cleaved selectively by various methods, most readily by reduction with tetrabutylammonium borohydride 9, to yield baccatin III (Structure 2) as the RO
H
o
O
OH
~ O OH (~COC6H5
2 R=Ac 3R=H diterpenoid component. Baccatin III can also be isolated from various Taxus species (such as T. baccatalO,11 and T. wallichianai2), and its analog 10-deacetylbaccatin IlI [10-DAB, (Structure 3)] can be obtained in an excellent yield of 0.1% from T. baccata needles 13. Baccatin III is significantly less active than taxol in both cytotoxicity assaysI and tubulin-assembly assaysTM, indicating the importance of the side chain for its activity. Baccatin III can, however, be converted back to taxol by coupling with an appropriately activated and protected side chain. Several methods have been developed for doing this, including direct coup-
ling of a protected baccatin III with a side chain protected as its ethoxyethyl derivative 13 or as its oxazolidine derivative is, and coupling of a protected baccatin III with a [3-1actami6-18. These methods have made it possible to elucidate more-detailed structure-activity rdationships of the side chain portion. Various taxol analogs that lack the 3'-phenyl group of the side chain are significantly less active than taxo119, and several analogs with substituted 3'-phenyl groups have also been prepared; all such analogs described to date have been less active than taxol2°-22. Analogs in which the side chain N-benzoyl group is replaced with other acyl groups have also been prepared, and one such analog, Taxotere ® (docetaxel), has therapeutic potential. Taxotere ® (Structure 4) has
HO (CHa)3OCONH
0
OH
O
O
C6H5~ (DH
"
X
~
O
OH (~COC6H5 4
an N-t-butoxycarbonyl group in place of the N-benzoyl group of taxol, and also lacks the 10-acetate group23; it is about five times as active as taxol against
TIBTECHJUNE1994(VOL12)
224
fOCblS taxol-resistant ceUs24, and is currently in clinical trials in both France and the USA. Other modifications at the N-benzoyl position have, however, yielded analogs with diminished activity compared with taxo120-22. Analogs in which the position and stereochemistry of the hydroxyl and benzoylamino groups of the side chain are altered have also been prepared 23, and all of these have been less active than taxol.
The northernperimeter The 'northern perimeter' portion of the taxol molecule comprises carbons 6-12, with oxygen functions at C-7, C-9 and C-10 (see Fig. 1). Acylation of the C-7 hydroxyl group 25, or its removal16,27, does not significantly reduce the activity of taxol. Similarly, removal of the 10-acetoxy group causes only a small reduction in activity28,29, and reduction of the C-9 carbonyl group to an oL-OH group causes only a slight increase in tubulin-assembly activity3°. An interesting rearrangement product with a cyclopropane ring bridging to the 7,8-position is almost as cytotoxic as taxoP 1. These results indicate that structural variations along the northern perimeter do not greatly affect the bioactivity of taxol, suggesting that this region of the molecule is not intimately involved in binding to microtubules.
The southernperimeter The 'southern perimeter' portion of the molecule includes C-14, and C-1-C-5, and contains oxygen functions at C-1, C-2 and C-4, and the unusual oxetane ring at C-4-C-5 (Fig. 1). This region appears to be crucial to taxol's activity, as structural changes have major effects on activity. Opening of the oxetane ring, either after oxidation at C-7 (Ref. 32) or with electrophilic reagents33, yields ring-opened products (e.g. Structure 5) that are much AcO
C6H5CONH
6
5
O
--
(~H
O
OH
~ -
"-,t
,, = ~u ~ OH ~COC6H 5
O A c
5 less active than taxol in both tubulin-assembly and cytotoxicity assays. The oxetane ring is thus a crucial component of the structure oftaxol. The same is also true of the benzoyloxy group at C-2, since deoxygenation at C-2 yields an inactive product 34. Several analogs with substituted benzoyl groups at C-2 have been prepared, and the interesting generalization can be made that m-substituted benzoyl derivatives are more active than their p-substituted analogs, and are often more active than taxol itself3s. TIBTECHJUNE1994 (VOL12)
The importance of the benzoyl group at C-2 makes sense in light of the fact that a m-stacking interaction between the phenyl group of the side chain and the C-2 benzoyl group has been proposed 36, although the difference between the m- and p-substituted benzoyl analogs indicates that steric factors must also play a part. The importance of the oxetane ring is surprising, in view of its relative chemical inertness, and suggests that its role may simply be to act as a lock to maintain the conformation of the diterpenoid ring system of taxoP 3.
The diterpenoid ring system Taxol can be converted to a rearranged product (Structure 6) with a contracted A-ring33. This A-norO
C6HsCONH
OH
O
coV.O OH
H 2C-,,--
-
~OCOC6H 5 6
taxol has a tubulin-assembly activity that is, surprisingly, only three times less than that of taxol, even though its cytotoxicity is reduced by orders of magnitude. A molecular-modeling comparison of taxol (Structure 1) and the A-nortaxol (Structure 6) showed that they have very similar shapes, thus providing a partial explanation of their similar tubulin-assembly activities.
The supply of taxol The development oftaxol for clinical use as an anticancer drug was delayed by two major problems (see above). The most difficult one to solve was the problem of supply, since taxol was initially available only from the bark of T. brevifolia in yields averaging 0.007% (1Kef. 37). The collection of large amounts of bark for clinical trials (27 000kg in 1989, for example) raised concerns about the impact of continued collection on the ecology of the US Pacific Northwest, in general, and on the survival of T. brevifolia in particular. The various approaches to solving this problem illustrate the different contributions that chemistry and biotechnology can make to this type of situation, as described below.
Isolation of taxolfrom Taxus species Isolation from the bark of Taxus species was the approach by which taxol was initially discovered and this approach has yielded nearly all the taxol available to date. Although the initial yield of taxol from T. brevifolia bark was only about 0.007%, this has now been doubled to 0.014% by a combination of improved isolation methods and improved bark-collection methods 37. Taxol can also be isolated from the
225
fOCUS leaves (or needles) of various Taxus species in yields comparable to the yield from bark 37. This is an important consideration since leaves are a renewable resource and, thus, the environmental objections to bark collection are avoided.
Partial synthesis of taxolfrom lO-deacetylbaccatinIll The taxol precursor 10-DAB (Structure 3) is available from the leaves of T. baccata and other yews in yields of at least 0.1% (Ref. 13). As noted earlier, several chemical methods have been developed to convert 10-DAB into taxol, and one of these (the [~-lactam approach, first developed by R. A. Holton) has been licensed by Bristol-Myers Squibb (Wallingford, CT, USA) and is being used for commercial production of taxol. A measure of the success of this approach is that Bristol-Myers Squibb announced in 1993 that it will no longer need to purchase taxol isolated from T. brevifolia bark and will end its contract with its supplier, Hauser Chemical Research (Boulder, CO, USA), in mid-1994. A major advantage of the partial-synthesis approach, as compared with direct isolation, is that it permits the synthesis of taxol analogs as well as taxol. Although most of the side chain analogs reported to date (with the notable exception of Taxotere ®) have been less active than taxol, it is very likely that other (as yet undisclosed or undiscovered) analogs will show improved activity, and will form the basis of secondgeneration taxol analogs.
callus cultures, thus complicating the purification process 40.
Taxol production from callus cultures of T. cuspidata and T. canadensis has also been demonstrated on a laboratory scale, with yields of 0.02% from 77. cuspidata callus reported 42. Suspension cultures of T. cuspidata have also been established and immobilized on glassfiber mats, yielding taxol in 0.012% yield (dryweight) 42. Subsequent manipulations of conditions doubled the yield of taxol from callus culture 43. Supplementation of the medium with phenylalanine promoted taxol biosynthesis without reducing callus growth 43 - an effect that is readily understandable as phenylalanine is one of the biosynthetic precursors of taxol in T. canadensis 44 and T. brevi)blia45. The production oftaxol from T. brevifolia callus culture has also been claimed by a Japanese group; they report yields of 0.05% (dry-weight) 46. In addition to the biosynthetic incorporation of phenylalanine reported above, acetate and mevalonate have also been shown to be biosynthetic precursors of taxol in T. cuspidata 44. In a study using T. brevifolia bark, phenylalanine and leucine were the best precursors, but acetate was also incorporated 47. Interestingly, a later study showed that only T. canadensis, T.floridana, and T. brevifolia from Montana (USA) incorporated acetate into taxol, while T. brevifolia from the US Pacific Northwest and eight other Taxus species did n o t i n c o r p o r a t e acetate 48.
Fungal production Plant tissue culture and biosynthesis Plant tissue-culture methods are an important method oftaxol production, arid offer what is arguably the next best approach (after partial synthesis) for largescale production. The first report of tissue-culture production oftaxol from T. brevifolia cultures was from the US Department of Agriculture (USDA)38: this discovery has been patented 39 and licensed to Phyton Catalytic (NY, USA). This company has been able to increase the yield oftaxol to 1-3 mg 1-1 ofsupernatant, using various off-the-shelf elicitors. In 1991, the company predicted that it would begin commercial production in 2-5 years4°; although, obviously, the more optimistic of these predictions is now history, the Phyton Catalytic method does have the significant advantage of yielding relatively pure taxol, thus greatly simplifying the isolation and purification process. Interestingly, not all cultures of T. brevifolia produce taxol; some callus cultures from bark, stem and needle tissues yielded no detectable taxol, although other taxane diterpenoids were detected 41. A second company, ESCAgenetics (CA, USA), has also announced plans to produce taxol using plant tissue culture. Although details of its proprietary 'phytoproduction system' are unavailable, it has been speculated that it uses hairy-root cultures produced with agrobacterial transforming agents 4°. The use of hairyroot cultures can result in fast growth with correspondingly increased production capacity; however, the product mix is usually more complex than with
In 1993, it was announced that taxol is produced by the fungus Taxomyces andreanae, found growing on one particular specimen of T brevifolia49. Although the yield of taxol was very low (24-50 ng 1-1), the genetic manipulation of fungi is achieved much more easily than that of plants. It is thus highly likely that strain improvement by classical techniques or by genetic engineering will result in gready improved production, even though the yield improvement needed to achieve this would be orders of magnitude greater than anything accomplished to date. If this can be done, however, it would open up the option of producing taxol by classical fermentation methods, at a much more modest cost than possible with plant tissue culture.
Total synthesis Mthough the partial synthesis of taxol is an attractive option, total synthesis is another matter altogether. The baccatin III portion of taxol is a complex tetracyclic system with many functional groups and stereochemical features, and it represents a synthetic challenge on which many of the world's leading synthetic organic chemists have embarked s. The first total syntheses of taxol have been achieved very recently by Holton and co-workers s°,51, and by Nicolaou et al. s2 and represent an enormous synthetic accomplishment. It is unlikely, however, that total synthesis will ever contribute significantly to the taxol supply, since the alternative methods described above are much more economical. TIBTECHJUNE 1994 (VOL12)
226
fOCUS Although total synthesis will not be important for the commercial production of taxol, it may well play a significant role in the commercial production of one or more second-generation taxol analogs. This is particularly likely if the best analogs prove either to lack some of the functionality of taxol, or to have modified functionality at other positions (since this would increase the difficulty of their preparation from either taxol or IO-DAB).
Taxol prodrugs The other major problem in the pharmaceutical development oftaxol has been its insolubility in water. Although this problem has been circumvented by the development of an emulsion formulation using Cremophor EL ® as the surfactant, this formulation is far from ideal. An improved formulation is thus an urgent need, and various groups have investigated the synthesis ofa prodrug (i.e. an inactive chemical derivative of a drug that is metabolized to yield the pharmacologically active drug) 7 form of taxol. It seems highly likely that a prodrug version of taxol will be developed for clinical use within the next year. Mechanism of action Taxol promotes the assembly of tubulin into microtubules 3 and, in the process, binds to the assembled microtubules in an approximately stoichiometric ratio with tubulin. An understanding of the binding oftaxol to microtubules would help in the development of improved taxol analogs, as such an understanding would facilitate the design of molecules that fit the binding site in a similar way. Information on the amino acid sequence of the binding site could, alternatively, be used to design binding-site mimics that could then be used as screens for detecting other compounds with taxoMike activity37. The most direct method of obtaining binding-site information is by photoaffinity labeling53. An important recent result is that an azido-labeled taxol photolabels the N-terminal 31 amino acids of f3-tubulin54; this provides the first clear evidence of the position of the taxol binding-site, and future work should lead to refined definition of the binding site. Future developments Our understanding of taxol and its activity has increased greatly over the past few years, from both a clinical perspective 55,56 and a medicinal chemistry point of view. The taxol supply problem has been solved, at least in the short term, by the development of the partial-synthesis approach, and attention has now turned to the development of improved analogs of taxol. One analog - Taxotere ® - has already been developed and is progressing through clinical trials57. The next analogs to be developed will almost certainly be prodrug forms of taxol and/or Taxotere ®. These will be followed by analogs that differ in other ways from taxol: analogs with modified side chains will probably be developed first, because their synthesis from TIBTECHJUNE 1994 (VOL 12)
/0-DAB is relatively straightforward; analogs with modified diterpenoid portions will probably provide improved bioactivity, and will thus be developed either simultaneously with, or shortly after, the sidechain analogs. Developments beyond the preparation of improved analogs are harder to predict reliably. It is, however, exciting to consider the possibility that taxol is simply a mimic of a natural cellular component that is involved in the regulation of mitosis s8. If this turns out to be the case, then the characterization of such a substance would open up an entire new class of chemical substances to pharmacological investigation.
References 1 Wani, M. C., Taylor, H. L., Wall, M. E., Coggon, P. and McPhail, A. T. (1971)J. Am. Chem. Soc. 93, 2325-2327 2 Suffness, M. and CordelI, G. A. (1985) in The Alkaloids, Vol. 25 (Brossi, A., ed.), pp. 1-369, Academic Press 3 Schi~ P. B., Fant, J. and Horwitz, S. B. (1979) Nature 277, 665-667 4 Weiss, R. B. et al. (1990).]. Clin. Oncol. 8, 1263-1268 5 McGuire, W. P. etal. (1989) Ann. Intern. Med. 111,273-279 6 Holmes, F. A. et al. (1991)J. Natl Cancer Inst. 83, 1797-1805 7 Kingston, D. G. I. (1991) Pharm. Ther. 52, 1-34 8 Kingston, D. G. I., Molinero, A. A. and Riraoldi, J. M. (1993) in Progress in the Chemistry of Organic Natural Products, VoI. 61 (Herz, W., Kirby, G. W., Moore, R. E., Steglich, W. and Tamm, C., eds), pp. 1-206, Springer-Verlag 9 Magri, N. F., Kingston, D. G. I., Jitrangsri, C. and Piccariello, T. (1986)J. Org. Chem. 51, 3239-3242 10 Della Casa de Marcano, D. P. and Halsall, T. G. (1975)J. Chem. Soc. Chem. Commun. 365-366 11 Stnilh, V. etaL (1984)J. Nat. Prod. 47, 131-137 12 Miller, R. W., Powell, R. G., Smith, C. R., Jr, Arnold, E. and Clardy, J. (1981)J. Org. Chem. 46, 146%1474 13 Denis, J-N., Greene, A. E., Gu8nard, D., Gu&itte-Voegelein, F., Mangatal, L. and Potier, P. (1988)J. Am. Chem. Soc. 110, 5917-5919 14 Lataste, H., Stnilh, V., Wright, M., Gu~nard, D. and Potier, P. (1984) Proc. Natl Acad. Sci. USA 81, 4090-4094 15 Commerqon, A., B&ard, D., Bernard, F. and Bourzat, J. D. (1992) Tetrahedron Lett. 33, 5185-5188 16 Georg, G. I., Chernvalhth, Z. S., Hamman, G. C. B., Hepperle, M. and Park, H. (1993) Bioorg. Med. Chem. Lett. 3, 2467-2470 17 Ojima, I. et aL (1992) Tetrahedron 48, 6985-7012 18 Holton, R. A. (1990) Chem. Abstr. 114, 164568q 19 Swindell, C. S., Krauss, N. E., Horwitz, S. B. and Ringel, I. (1991) J. IVied. Chem. 34, 1176-1184 20 Georg, G. I., Cheruvallath, Z. S., Himes, R. H. and Mejillano, M. R. (1992) Bioorg. Med. Chem. Lett. 2, 295-298 21 Georg, G. I., Cheruvallath, Z. S., Himes, R. H. and Mejillano, M. R. (1992) Bioorg. Med. Chem. Lett. 2, 1751-1754 22 Georg, G. I., Chernvallath, Z. S., Himes, R. H., MejiUano, M. R. and Burke, C. T. (1992)J. Med. Chem. 35, 4230-4237 23 GuSritte-Voegelein, F., Gutnard, D., Lavelle, F., Le Goff, M-T., Mangatal, L. and Potier, P. (1991)J. Med. Chem. 34, 992-998 24 Ringel, I. and Horwitz, S. B. (1991)J. Natl Cancerlnst. 83, 288-291 25 Mellado, W., Magri, N. F., Kingston, D. G. I., Garcia-Arenas, R., Orr, G. A. and Horwitz, S. B. (1984) Biochem. Biophys. Res. Commun. 124, 329-336 26 Chen, S. H., Huang, S., Kant, J , Fairchild, C., Wei,J. M. and Farina, V. (1993)J. Org. Chem. 58, 5028-5029 27 Chaudhary, A. G., Rimoldi, J. M. and Kingston, D. G. I. (1993) J. Org. Chem. 58, 3798-3799 28 Chandhary, A. G. and Kingston, D. G. I. (1993) Tetrahedron Lett. 34, 4921-4924 29 Chen, S., Fairchild, C., Mamber, S. W. and Farina, V. (1993)J. Org. Chem. 58, 2927-2928 30 Klein, L. L. (1993) ~Ttrahedron Lett. 34, 2047-2050
227
fOCUS 31 Chen, S-H., Huang, S., Wei, J. and Farina, V. (1993)./. Org. Chem. 58, 4520-4521 32 Magri, N. F. and Kingston, D. G. I. (1986)J. Org. Chem. 51,797-802 33 Samaranayake, G., Magri, N. F., Jitrangsri, C. and Kingston, D. G. I. (1991)./. Org. Chem. 56, 5114-5119 34 Chen, S-H., Wei, J-M. and Farina, V. (1993) Tetrahedron Lett. 34, 3205-3206 35 Chaudhary, A. G. et al. (I994)J. Am. Chem. Soc. 116, 4097-4098 36 Williams, H.J. et al. (1993) Tetrahedron 49, 6545-6560 37 Cragg, G. M., Schepartz, S. A., Suffness, M. and Grever, M. R. (1993)J. Nat. Prod. 56, 1657-1668 38 Christen, A. A., Bland, J. and Gibson, D. M. (1989) Proc. Am. Assoc. Cancer Res. 30, 566 39 Christen, A. A., Gibson, D. M., and Bland, J. (1991) US Patent 5019504 40 Edgington, S. M. (1991) Bio/Technology 9, 933-938 41 Gibson, D. M., Ketchum, Ik. E. B., Vance, N. C. and Christen, A. A. (1993) Plant Cell Rep. 12, 479-482 42 Fett-Neto, A. G., DiCosmo, F., Reynolds, W. F. and Sakata, K. (1992) Bio/Technology 10, 1572-1575 43 Fett-Neto, A. G., Melanson, s.J., Sakata, K. and DiCosmo, F. (1993) Bio/Technology 11,731-734
44 Zamir, L. O., Nedea, M. E. and Garneau, F. X. (1992) Tetrahedron Lett. 33, 5235-5236 45 Fleming, P. E., Mocek, U. and Floss, H. G. (1993)J. Am. Chem. Soc. 115, 805-807 46 Saito, K., Ohashi, H., Hibi, M. and Tahara, M. (1992) Chem. Abstr. 117, 190280w 47 Strobel, G. A., Stierle, A. and van Kuijk, F.J.G.M. (1992) Plant &i. 84, 65-74 48 Strobel, G. A., Stierle, A. and Hess, W. M. (1993) Plant Sci. 92, 1 12 49 Stierle, A., Strobel, G. and Stierle, D. (1993) Science260, 214-216 50 Holton, lq,. A. et al. (1994)J. Am. Chem. Soc. 116, 1597-1598 51 Holton, lq,. A. et al. (1994)J. Am. Chem. Soc. 116, 1599-1600 52 Nicolaou, K. C. et al. (1994) Nature 367, 630-634 53 P,.imoldi, J. M., Kingston, D. G. I., Chaudhary, A. G., Samaranayake, G., Grovel S. and Hamel, E. (1993)J. Nat. Prod. 56, 1313-1330 54 Rao, S. et al. (1994)J. Biol. Chem. 269, 3132-3134 55 Slichenmyer, W.J. and Von Hoff, D. D. (1991) Anti-cancerDrugs 2, 519-530 56 P, owinsky, E. K. et al. (1993) Semin. Oncol. 20 4, (Suppl. 3), 1-60 57 Gu6nard, D., Gu6ritte-Voegelein, F. and Potier, P. (1993) Acc. Chem. Res. 26, 160-167 58 Manfi'edi,J.J. and Horwitz, S. B. (1984) Pharmacol. Ther. 25, 83-125
Exploiting altered glycosylation patterns in cancer: progress and challenges in diagnosis and therapy Joyce Taylor-Papadimitriouand AgamemnonA. Epenetos In this review, we describe the changes in glycosylation and expression of mucins, in particular, polymorphic epithelial mucin (PEM), the product of the MUC1 gene in tumours and normal tissues. In addition, some of the areas where exploitation of altered glycosylation patterns in tumour mucins can be used for the better understanding of the disease process and be applied for in vivo diagnosis and therapy are addressed. It has been known for many years that the surface molecules of cancer cells can undergo changes in their glycosylation profile. In recent years, however, these changes have been defined in more detail, and it is now possible to take a more directed approach to exploiting them in the diagnosis and treatment of j . Taylor-Papadimitriou is at the Epithelial Cell Biology Laboratory, Imperial Cancer Research Fund, 44 Lincoln's Inn Field, London, U K W C 2 A 3PX. A . A . Epenetos is at the Tumour Targeting Laboratory, I C R F Clinical Oncology Unit, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London, U K W 1 2 OHS. © 1994, Elsevier Science Ltd
cancer. Progress in this area has been aided by the molecular characterization of high-molecular-weight, O-linked glycoproteins (known as mucins). These proteins, which are expressed by epithelial cells and the carcinomas that develop from them, carry multiple oligosacccharide side chains covalently attached to the hydroxy amino acids serine and threonine. This article is primarily concerned with mucin molecules and, in particular, the product of the MUC1gene (the polymorphic epithelial mucin, PEM), which has been analysed in the most detail. The potential use of PEMbased immunogens and PEM-reactive antibodies in the immunotherapy of carcinomas is also discussed. TIBTECHJUNE 1994 (VOL 12)
Taxol: the chemistry and structure-activity relationships of a novel anticancer agent David G. I. Kingston Taxol is an exciting new anticancer drug, showing clinical activity against ovarian and breast cancer. Its development as a clinically useful drug has involved major efforts to overcome the supply problem; this has now been done, and the focus of interest has moved to the development of improved analogs of the drug. Recent notable achievements include the first total synthesis of taxol, and the first indications of its binding site on tubulin. For many centuries, the yew tree was regarded as the tree of death, and its toxic constituents provided a death potion in Julius Caesar's time. It is thus both ironic and encouraging that tiffs same tree should now give cause for hope in the fight against cancer. The diterpenoid taxol (Structure 1) was isolated
AcO IB"4 1 C6HsCON H
O
T ~"
sO
2~,~/__/~-~9 ~'~
OH
I
3[
6 ],5
)i. OH
~COCsH 5
1 from the Western Yew (Taxus brevifolia) by Monroe Wall and his collaborators in the late 1960s, and its structure was published in 19711. Although taxol showed good activity in the cytotoxicity assays and in vivo antileukemic assays then in use, it was not given a high priority for development because of two obvious obstacles to its success as an anticancer drug: (1) its very low solubility in water, making it difficult to formMate; and (2) its very low yield from the bark o f T. brevifolia. Fortunately, additional biological testing did continue, and in the mid-1970s, taxol was found to show activity in some new bioassays, including the B 16 melanoma assay and three human-tumorxenograft assays in nude mice 2. Based on these new data, taxol was advanced into preclinical development by the US National Cancer Institute (NCI) in 1977. D. G. L Kingston b at the Department of Chemistry, Virginia Polytechnic Institute and State University, BIacksburg, VA 24061-0212, USA. TIBTECHJUNE1994(VOL12)
An important discovery in •979 reinforced the earlier decision to investigate taxol further, and generated significant interest in taxol in the biological community. In that year, Susan Horwitz and her collaborators made the important discovery that taxol promotes polymerization o f the cellular protein tubulin, causing it to assemble into stable microtubules 3. It is believed that this mechanism is responsible for taxol's action as an antimitotic drug, and taxol has thus become a valuable biochemical tool for studying mitosis. Taxol entered Phase I clinical trials in 1983, but immediately ran into some problems related to its formulation. As noted earlier, it is very insoluble in water, and it was eventually formulated as an emulsion with Cremophor EL ®, a polyethoxylated castor oil. As taxol must be given at relatively high dosages, large amounts of Cremophor EL ® were required; this caused problems with allergic reactions in some patients, including one death 4. Fortunately, these problems were overcome by lengthening the infusion period and premedicating patients with glucocorticoids and antihistamines. Phase II trials were initiated in 1985 and these proved to be very successful: taxol was found to have excellent activity against drug-refractory ovarian cancer5 and against breast cancer 6. It was approved by the US Food and Drug Administration (FDA) for the treatment o f ovarian cancer in 1992.
Chemistry and structure-activity relationships of taxol Extensive chemical characterization o f taxol has been carried out (see Ret~ 7, 8 for review). These studies have provided a still-incomplete but, nevertheless, substantial body of information on structure-activity relationships, which is summarized below and in Fig. 1. © 1994, ElsevierScienceLtd
223
fOCUS
Acetyl or acetoxy group may be removed without significant loss of activity C
Reduction improves activity slightly /
~ ¢/ O 19 OH
AcO N-awl group. ~ O required /~~.,,~
18 -NH
1
~
~12./~i
~
II
]
--
.OH
~
~--';~-7, -vnenylgroup or a close analog required
6 81
~-
I
~
"~.~"~'L~I 9 ~ ! ( x ~=~N./" O la (~w - Acu ~
~1
~ //"
May be esterified, epimerized or removedwithout significantloss of activity
4 C~".O
~ t -Free 2'-hydroxyl group, or a hydrolysable ester thereof required
IO~e~an$.rlng I required fo I activity
="
~~ # ( ~
~
'
~
i[ Removalof acetate ] [ reduces activity slightly
-!
Benzoyloxygroup essential; certain substituted groups have improved activity
Figure 1
Structure-activityrelationshipsoftaxol. The side chain The N-benzoyl-13-phenylisoserine side chain of taxol can be cleaved selectively by various methods, most readily by reduction with tetrabutylammonium borohydride 9, to yield baccatin III (Structure 2) as the RO
H
o
O
OH
~ O OH (~COC6H5
2 R=Ac 3R=H diterpenoid component. Baccatin III can also be isolated from various Taxus species (such as T. baccatalO,11 and T. wallichianai2), and its analog 10-deacetylbaccatin IlI [10-DAB, (Structure 3)] can be obtained in an excellent yield of 0.1% from T. baccata needles 13. Baccatin III is significantly less active than taxol in both cytotoxicity assaysI and tubulin-assembly assaysTM, indicating the importance of the side chain for its activity. Baccatin III can, however, be converted back to taxol by coupling with an appropriately activated and protected side chain. Several methods have been developed for doing this, including direct coup-
ling of a protected baccatin III with a side chain protected as its ethoxyethyl derivative 13 or as its oxazolidine derivative is, and coupling of a protected baccatin III with a [3-1actami6-18. These methods have made it possible to elucidate more-detailed structure-activity rdationships of the side chain portion. Various taxol analogs that lack the 3'-phenyl group of the side chain are significantly less active than taxo119, and several analogs with substituted 3'-phenyl groups have also been prepared; all such analogs described to date have been less active than taxol2°-22. Analogs in which the side chain N-benzoyl group is replaced with other acyl groups have also been prepared, and one such analog, Taxotere ® (docetaxel), has therapeutic potential. Taxotere ® (Structure 4) has
HO (CHa)3OCONH
0
OH
O
O
C6H5~ (DH
"
X
~
O
OH (~COC6H5 4
an N-t-butoxycarbonyl group in place of the N-benzoyl group of taxol, and also lacks the 10-acetate group23; it is about five times as active as taxol against
TIBTECHJUNE1994(VOL12)
224
fOCblS taxol-resistant ceUs24, and is currently in clinical trials in both France and the USA. Other modifications at the N-benzoyl position have, however, yielded analogs with diminished activity compared with taxo120-22. Analogs in which the position and stereochemistry of the hydroxyl and benzoylamino groups of the side chain are altered have also been prepared 23, and all of these have been less active than taxol.
The northernperimeter The 'northern perimeter' portion of the taxol molecule comprises carbons 6-12, with oxygen functions at C-7, C-9 and C-10 (see Fig. 1). Acylation of the C-7 hydroxyl group 25, or its removal16,27, does not significantly reduce the activity of taxol. Similarly, removal of the 10-acetoxy group causes only a small reduction in activity28,29, and reduction of the C-9 carbonyl group to an oL-OH group causes only a slight increase in tubulin-assembly activity3°. An interesting rearrangement product with a cyclopropane ring bridging to the 7,8-position is almost as cytotoxic as taxoP 1. These results indicate that structural variations along the northern perimeter do not greatly affect the bioactivity of taxol, suggesting that this region of the molecule is not intimately involved in binding to microtubules.
The southernperimeter The 'southern perimeter' portion of the molecule includes C-14, and C-1-C-5, and contains oxygen functions at C-1, C-2 and C-4, and the unusual oxetane ring at C-4-C-5 (Fig. 1). This region appears to be crucial to taxol's activity, as structural changes have major effects on activity. Opening of the oxetane ring, either after oxidation at C-7 (Ref. 32) or with electrophilic reagents33, yields ring-opened products (e.g. Structure 5) that are much AcO
C6H5CONH
6
5
O
--
(~H
O
OH
~ -
"-,t
,, = ~u ~ OH ~COC6H 5
O A c
5 less active than taxol in both tubulin-assembly and cytotoxicity assays. The oxetane ring is thus a crucial component of the structure oftaxol. The same is also true of the benzoyloxy group at C-2, since deoxygenation at C-2 yields an inactive product 34. Several analogs with substituted benzoyl groups at C-2 have been prepared, and the interesting generalization can be made that m-substituted benzoyl derivatives are more active than their p-substituted analogs, and are often more active than taxol itself3s. TIBTECHJUNE1994 (VOL12)
The importance of the benzoyl group at C-2 makes sense in light of the fact that a m-stacking interaction between the phenyl group of the side chain and the C-2 benzoyl group has been proposed 36, although the difference between the m- and p-substituted benzoyl analogs indicates that steric factors must also play a part. The importance of the oxetane ring is surprising, in view of its relative chemical inertness, and suggests that its role may simply be to act as a lock to maintain the conformation of the diterpenoid ring system of taxoP 3.
The diterpenoid ring system Taxol can be converted to a rearranged product (Structure 6) with a contracted A-ring33. This A-norO
C6HsCONH
OH
O
coV.O OH
H 2C-,,--
-
~OCOC6H 5 6
taxol has a tubulin-assembly activity that is, surprisingly, only three times less than that of taxol, even though its cytotoxicity is reduced by orders of magnitude. A molecular-modeling comparison of taxol (Structure 1) and the A-nortaxol (Structure 6) showed that they have very similar shapes, thus providing a partial explanation of their similar tubulin-assembly activities.
The supply of taxol The development oftaxol for clinical use as an anticancer drug was delayed by two major problems (see above). The most difficult one to solve was the problem of supply, since taxol was initially available only from the bark of T. brevifolia in yields averaging 0.007% (1Kef. 37). The collection of large amounts of bark for clinical trials (27 000kg in 1989, for example) raised concerns about the impact of continued collection on the ecology of the US Pacific Northwest, in general, and on the survival of T. brevifolia in particular. The various approaches to solving this problem illustrate the different contributions that chemistry and biotechnology can make to this type of situation, as described below.
Isolation of taxolfrom Taxus species Isolation from the bark of Taxus species was the approach by which taxol was initially discovered and this approach has yielded nearly all the taxol available to date. Although the initial yield of taxol from T. brevifolia bark was only about 0.007%, this has now been doubled to 0.014% by a combination of improved isolation methods and improved bark-collection methods 37. Taxol can also be isolated from the
225
fOCUS leaves (or needles) of various Taxus species in yields comparable to the yield from bark 37. This is an important consideration since leaves are a renewable resource and, thus, the environmental objections to bark collection are avoided.
Partial synthesis of taxolfrom lO-deacetylbaccatinIll The taxol precursor 10-DAB (Structure 3) is available from the leaves of T. baccata and other yews in yields of at least 0.1% (Ref. 13). As noted earlier, several chemical methods have been developed to convert 10-DAB into taxol, and one of these (the [~-lactam approach, first developed by R. A. Holton) has been licensed by Bristol-Myers Squibb (Wallingford, CT, USA) and is being used for commercial production of taxol. A measure of the success of this approach is that Bristol-Myers Squibb announced in 1993 that it will no longer need to purchase taxol isolated from T. brevifolia bark and will end its contract with its supplier, Hauser Chemical Research (Boulder, CO, USA), in mid-1994. A major advantage of the partial-synthesis approach, as compared with direct isolation, is that it permits the synthesis of taxol analogs as well as taxol. Although most of the side chain analogs reported to date (with the notable exception of Taxotere ®) have been less active than taxol, it is very likely that other (as yet undisclosed or undiscovered) analogs will show improved activity, and will form the basis of secondgeneration taxol analogs.
callus cultures, thus complicating the purification process 40.
Taxol production from callus cultures of T. cuspidata and T. canadensis has also been demonstrated on a laboratory scale, with yields of 0.02% from 77. cuspidata callus reported 42. Suspension cultures of T. cuspidata have also been established and immobilized on glassfiber mats, yielding taxol in 0.012% yield (dryweight) 42. Subsequent manipulations of conditions doubled the yield of taxol from callus culture 43. Supplementation of the medium with phenylalanine promoted taxol biosynthesis without reducing callus growth 43 - an effect that is readily understandable as phenylalanine is one of the biosynthetic precursors of taxol in T. canadensis 44 and T. brevi)blia45. The production oftaxol from T. brevifolia callus culture has also been claimed by a Japanese group; they report yields of 0.05% (dry-weight) 46. In addition to the biosynthetic incorporation of phenylalanine reported above, acetate and mevalonate have also been shown to be biosynthetic precursors of taxol in T. cuspidata 44. In a study using T. brevifolia bark, phenylalanine and leucine were the best precursors, but acetate was also incorporated 47. Interestingly, a later study showed that only T. canadensis, T.floridana, and T. brevifolia from Montana (USA) incorporated acetate into taxol, while T. brevifolia from the US Pacific Northwest and eight other Taxus species did n o t i n c o r p o r a t e acetate 48.
Fungal production Plant tissue culture and biosynthesis Plant tissue-culture methods are an important method oftaxol production, arid offer what is arguably the next best approach (after partial synthesis) for largescale production. The first report of tissue-culture production oftaxol from T. brevifolia cultures was from the US Department of Agriculture (USDA)38: this discovery has been patented 39 and licensed to Phyton Catalytic (NY, USA). This company has been able to increase the yield oftaxol to 1-3 mg 1-1 ofsupernatant, using various off-the-shelf elicitors. In 1991, the company predicted that it would begin commercial production in 2-5 years4°; although, obviously, the more optimistic of these predictions is now history, the Phyton Catalytic method does have the significant advantage of yielding relatively pure taxol, thus greatly simplifying the isolation and purification process. Interestingly, not all cultures of T. brevifolia produce taxol; some callus cultures from bark, stem and needle tissues yielded no detectable taxol, although other taxane diterpenoids were detected 41. A second company, ESCAgenetics (CA, USA), has also announced plans to produce taxol using plant tissue culture. Although details of its proprietary 'phytoproduction system' are unavailable, it has been speculated that it uses hairy-root cultures produced with agrobacterial transforming agents 4°. The use of hairyroot cultures can result in fast growth with correspondingly increased production capacity; however, the product mix is usually more complex than with
In 1993, it was announced that taxol is produced by the fungus Taxomyces andreanae, found growing on one particular specimen of T brevifolia49. Although the yield of taxol was very low (24-50 ng 1-1), the genetic manipulation of fungi is achieved much more easily than that of plants. It is thus highly likely that strain improvement by classical techniques or by genetic engineering will result in gready improved production, even though the yield improvement needed to achieve this would be orders of magnitude greater than anything accomplished to date. If this can be done, however, it would open up the option of producing taxol by classical fermentation methods, at a much more modest cost than possible with plant tissue culture.
Total synthesis Mthough the partial synthesis of taxol is an attractive option, total synthesis is another matter altogether. The baccatin III portion of taxol is a complex tetracyclic system with many functional groups and stereochemical features, and it represents a synthetic challenge on which many of the world's leading synthetic organic chemists have embarked s. The first total syntheses of taxol have been achieved very recently by Holton and co-workers s°,51, and by Nicolaou et al. s2 and represent an enormous synthetic accomplishment. It is unlikely, however, that total synthesis will ever contribute significantly to the taxol supply, since the alternative methods described above are much more economical. TIBTECHJUNE 1994 (VOL12)
226
fOCUS Although total synthesis will not be important for the commercial production of taxol, it may well play a significant role in the commercial production of one or more second-generation taxol analogs. This is particularly likely if the best analogs prove either to lack some of the functionality of taxol, or to have modified functionality at other positions (since this would increase the difficulty of their preparation from either taxol or IO-DAB).
Taxol prodrugs The other major problem in the pharmaceutical development oftaxol has been its insolubility in water. Although this problem has been circumvented by the development of an emulsion formulation using Cremophor EL ® as the surfactant, this formulation is far from ideal. An improved formulation is thus an urgent need, and various groups have investigated the synthesis ofa prodrug (i.e. an inactive chemical derivative of a drug that is metabolized to yield the pharmacologically active drug) 7 form of taxol. It seems highly likely that a prodrug version of taxol will be developed for clinical use within the next year. Mechanism of action Taxol promotes the assembly of tubulin into microtubules 3 and, in the process, binds to the assembled microtubules in an approximately stoichiometric ratio with tubulin. An understanding of the binding oftaxol to microtubules would help in the development of improved taxol analogs, as such an understanding would facilitate the design of molecules that fit the binding site in a similar way. Information on the amino acid sequence of the binding site could, alternatively, be used to design binding-site mimics that could then be used as screens for detecting other compounds with taxoMike activity37. The most direct method of obtaining binding-site information is by photoaffinity labeling53. An important recent result is that an azido-labeled taxol photolabels the N-terminal 31 amino acids of f3-tubulin54; this provides the first clear evidence of the position of the taxol binding-site, and future work should lead to refined definition of the binding site. Future developments Our understanding of taxol and its activity has increased greatly over the past few years, from both a clinical perspective 55,56 and a medicinal chemistry point of view. The taxol supply problem has been solved, at least in the short term, by the development of the partial-synthesis approach, and attention has now turned to the development of improved analogs of taxol. One analog - Taxotere ® - has already been developed and is progressing through clinical trials57. The next analogs to be developed will almost certainly be prodrug forms of taxol and/or Taxotere ®. These will be followed by analogs that differ in other ways from taxol: analogs with modified side chains will probably be developed first, because their synthesis from TIBTECHJUNE 1994 (VOL 12)
/0-DAB is relatively straightforward; analogs with modified diterpenoid portions will probably provide improved bioactivity, and will thus be developed either simultaneously with, or shortly after, the sidechain analogs. Developments beyond the preparation of improved analogs are harder to predict reliably. It is, however, exciting to consider the possibility that taxol is simply a mimic of a natural cellular component that is involved in the regulation of mitosis s8. If this turns out to be the case, then the characterization of such a substance would open up an entire new class of chemical substances to pharmacological investigation.
References 1 Wani, M. C., Taylor, H. L., Wall, M. E., Coggon, P. and McPhail, A. T. (1971)J. Am. Chem. Soc. 93, 2325-2327 2 Suffness, M. and CordelI, G. A. (1985) in The Alkaloids, Vol. 25 (Brossi, A., ed.), pp. 1-369, Academic Press 3 Schi~ P. B., Fant, J. and Horwitz, S. B. (1979) Nature 277, 665-667 4 Weiss, R. B. et al. (1990).]. Clin. Oncol. 8, 1263-1268 5 McGuire, W. P. etal. (1989) Ann. Intern. Med. 111,273-279 6 Holmes, F. A. et al. (1991)J. Natl Cancer Inst. 83, 1797-1805 7 Kingston, D. G. I. (1991) Pharm. Ther. 52, 1-34 8 Kingston, D. G. I., Molinero, A. A. and Riraoldi, J. M. (1993) in Progress in the Chemistry of Organic Natural Products, VoI. 61 (Herz, W., Kirby, G. W., Moore, R. E., Steglich, W. and Tamm, C., eds), pp. 1-206, Springer-Verlag 9 Magri, N. F., Kingston, D. G. I., Jitrangsri, C. and Piccariello, T. (1986)J. Org. Chem. 51, 3239-3242 10 Della Casa de Marcano, D. P. and Halsall, T. G. (1975)J. Chem. Soc. Chem. Commun. 365-366 11 Stnilh, V. etaL (1984)J. Nat. Prod. 47, 131-137 12 Miller, R. W., Powell, R. G., Smith, C. R., Jr, Arnold, E. and Clardy, J. (1981)J. Org. Chem. 46, 146%1474 13 Denis, J-N., Greene, A. E., Gu8nard, D., Gu&itte-Voegelein, F., Mangatal, L. and Potier, P. (1988)J. Am. Chem. Soc. 110, 5917-5919 14 Lataste, H., Stnilh, V., Wright, M., Gu~nard, D. and Potier, P. (1984) Proc. Natl Acad. Sci. USA 81, 4090-4094 15 Commerqon, A., B&ard, D., Bernard, F. and Bourzat, J. D. (1992) Tetrahedron Lett. 33, 5185-5188 16 Georg, G. I., Chernvalhth, Z. S., Hamman, G. C. B., Hepperle, M. and Park, H. (1993) Bioorg. Med. Chem. Lett. 3, 2467-2470 17 Ojima, I. et aL (1992) Tetrahedron 48, 6985-7012 18 Holton, R. A. (1990) Chem. Abstr. 114, 164568q 19 Swindell, C. S., Krauss, N. E., Horwitz, S. B. and Ringel, I. (1991) J. IVied. Chem. 34, 1176-1184 20 Georg, G. I., Cheruvallath, Z. S., Himes, R. H. and Mejillano, M. R. (1992) Bioorg. Med. Chem. Lett. 2, 295-298 21 Georg, G. I., Cheruvallath, Z. S., Himes, R. H. and Mejillano, M. R. (1992) Bioorg. Med. Chem. Lett. 2, 1751-1754 22 Georg, G. I., Chernvallath, Z. S., Himes, R. H., MejiUano, M. R. and Burke, C. T. (1992)J. Med. Chem. 35, 4230-4237 23 GuSritte-Voegelein, F., Gutnard, D., Lavelle, F., Le Goff, M-T., Mangatal, L. and Potier, P. (1991)J. Med. Chem. 34, 992-998 24 Ringel, I. and Horwitz, S. B. (1991)J. Natl Cancerlnst. 83, 288-291 25 Mellado, W., Magri, N. F., Kingston, D. G. I., Garcia-Arenas, R., Orr, G. A. and Horwitz, S. B. (1984) Biochem. Biophys. Res. Commun. 124, 329-336 26 Chen, S. H., Huang, S., Kant, J , Fairchild, C., Wei,J. M. and Farina, V. (1993)J. Org. Chem. 58, 5028-5029 27 Chaudhary, A. G., Rimoldi, J. M. and Kingston, D. G. I. (1993) J. Org. Chem. 58, 3798-3799 28 Chandhary, A. G. and Kingston, D. G. I. (1993) Tetrahedron Lett. 34, 4921-4924 29 Chen, S., Fairchild, C., Mamber, S. W. and Farina, V. (1993)J. Org. Chem. 58, 2927-2928 30 Klein, L. L. (1993) ~Ttrahedron Lett. 34, 2047-2050
227
fOCUS 31 Chen, S-H., Huang, S., Wei, J. and Farina, V. (1993)./. Org. Chem. 58, 4520-4521 32 Magri, N. F. and Kingston, D. G. I. (1986)J. Org. Chem. 51,797-802 33 Samaranayake, G., Magri, N. F., Jitrangsri, C. and Kingston, D. G. I. (1991)./. Org. Chem. 56, 5114-5119 34 Chen, S-H., Wei, J-M. and Farina, V. (1993) Tetrahedron Lett. 34, 3205-3206 35 Chaudhary, A. G. et al. (I994)J. Am. Chem. Soc. 116, 4097-4098 36 Williams, H.J. et al. (1993) Tetrahedron 49, 6545-6560 37 Cragg, G. M., Schepartz, S. A., Suffness, M. and Grever, M. R. (1993)J. Nat. Prod. 56, 1657-1668 38 Christen, A. A., Bland, J. and Gibson, D. M. (1989) Proc. Am. Assoc. Cancer Res. 30, 566 39 Christen, A. A., Gibson, D. M., and Bland, J. (1991) US Patent 5019504 40 Edgington, S. M. (1991) Bio/Technology 9, 933-938 41 Gibson, D. M., Ketchum, Ik. E. B., Vance, N. C. and Christen, A. A. (1993) Plant Cell Rep. 12, 479-482 42 Fett-Neto, A. G., DiCosmo, F., Reynolds, W. F. and Sakata, K. (1992) Bio/Technology 10, 1572-1575 43 Fett-Neto, A. G., Melanson, s.J., Sakata, K. and DiCosmo, F. (1993) Bio/Technology 11,731-734
44 Zamir, L. O., Nedea, M. E. and Garneau, F. X. (1992) Tetrahedron Lett. 33, 5235-5236 45 Fleming, P. E., Mocek, U. and Floss, H. G. (1993)J. Am. Chem. Soc. 115, 805-807 46 Saito, K., Ohashi, H., Hibi, M. and Tahara, M. (1992) Chem. Abstr. 117, 190280w 47 Strobel, G. A., Stierle, A. and van Kuijk, F.J.G.M. (1992) Plant &i. 84, 65-74 48 Strobel, G. A., Stierle, A. and Hess, W. M. (1993) Plant Sci. 92, 1 12 49 Stierle, A., Strobel, G. and Stierle, D. (1993) Science260, 214-216 50 Holton, lq,. A. et al. (1994)J. Am. Chem. Soc. 116, 1597-1598 51 Holton, lq,. A. et al. (1994)J. Am. Chem. Soc. 116, 1599-1600 52 Nicolaou, K. C. et al. (1994) Nature 367, 630-634 53 P,.imoldi, J. M., Kingston, D. G. I., Chaudhary, A. G., Samaranayake, G., Grovel S. and Hamel, E. (1993)J. Nat. Prod. 56, 1313-1330 54 Rao, S. et al. (1994)J. Biol. Chem. 269, 3132-3134 55 Slichenmyer, W.J. and Von Hoff, D. D. (1991) Anti-cancerDrugs 2, 519-530 56 P, owinsky, E. K. et al. (1993) Semin. Oncol. 20 4, (Suppl. 3), 1-60 57 Gu6nard, D., Gu6ritte-Voegelein, F. and Potier, P. (1993) Acc. Chem. Res. 26, 160-167 58 Manfi'edi,J.J. and Horwitz, S. B. (1984) Pharmacol. Ther. 25, 83-125
Exploiting altered glycosylation patterns in cancer: progress and challenges in diagnosis and therapy Joyce Taylor-Papadimitriouand AgamemnonA. Epenetos In this review, we describe the changes in glycosylation and expression of mucins, in particular, polymorphic epithelial mucin (PEM), the product of the MUC1 gene in tumours and normal tissues. In addition, some of the areas where exploitation of altered glycosylation patterns in tumour mucins can be used for the better understanding of the disease process and be applied for in vivo diagnosis and therapy are addressed. It has been known for many years that the surface molecules of cancer cells can undergo changes in their glycosylation profile. In recent years, however, these changes have been defined in more detail, and it is now possible to take a more directed approach to exploiting them in the diagnosis and treatment of j . Taylor-Papadimitriou is at the Epithelial Cell Biology Laboratory, Imperial Cancer Research Fund, 44 Lincoln's Inn Field, London, U K W C 2 A 3PX. A . A . Epenetos is at the Tumour Targeting Laboratory, I C R F Clinical Oncology Unit, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London, U K W 1 2 OHS. © 1994, Elsevier Science Ltd
cancer. Progress in this area has been aided by the molecular characterization of high-molecular-weight, O-linked glycoproteins (known as mucins). These proteins, which are expressed by epithelial cells and the carcinomas that develop from them, carry multiple oligosacccharide side chains covalently attached to the hydroxy amino acids serine and threonine. This article is primarily concerned with mucin molecules and, in particular, the product of the MUC1gene (the polymorphic epithelial mucin, PEM), which has been analysed in the most detail. The potential use of PEMbased immunogens and PEM-reactive antibodies in the immunotherapy of carcinomas is also discussed. TIBTECHJUNE 1994 (VOL 12)