OM-MA (CA125) and
Ovarian Cancer

Ovarian cancer is relatively common, and associated with high mortality rates: approximately one woman in 70 develops this form of cancer, which is the fourth leading cause of cancer deaths in women.¹ There are known risk factors--including family history, nulliparity, early menarche, late menopause, and advanced age--but the disease is frequently described as a "silent" cancer because clinical symptoms are often not apparent until an advanced stage and because, to date, no effective protocol for early detection has emerged.

During the past 20 years or so, we have seen several notable advances in treatment, involving new chemotherapeutic agents and aggressive surgery; and the clinical laboratory has played an increasingly prominent role in the fight against ovarian cancer. The statistics suggest that some progress is being made. Although overall age-adjusted mortality rates have changed little in the US during this time, there has been a relative decrease in the rates for women under 65 years of age, notably in the 45- to 55-year age bracket.¹

The laboratory's role
Several tumor markers have been evaluated for their ability to aid in the detection and management of ovarian cancer, but the one most widely used is still CA125 (OM-MA)--a marker which meta-analysis suggests may also be useful in connection with endometriosis.^2,3

CA125 has proved especially valuable to oncologists in managing the treatment of ovarian cancer patients. The marker has prognostic value both alone and in combination with interleukin-2 receptor-alpha (IL-2R alpha).⁴ An elevated CA125 level following surgical removal of the tumor or chemotherapy is a reliable indicator of persistent or recurrent disease. In addition, it has been reported that elevated CA125 levels before the start of a third chemotherapy course are related to poor prognosis, as is a CA125 half-life of more than 20 days.⁵

The optimal role for CA125 determinations in screening, however, remains to be established. There is general agreement that, except possibly in high-risk groups, CA125 measurements should not be used on their own to screen for ovarian cancer, especially in premenopausal women, as elevated CA125 levels have been encountered in a small percentage of healthy individuals and in patients with nonmalignant conditions such as pericarditis, first trimester pregnancy, endometriosis, tuberculosis and diseases with benign ascites. (Elevated levels can also be found in patients with nonovarian malignancies, such as pancreatic cancer, metastatic cancers of other sites of the gastrointestinal tract, and lung cancer.)

Nevertheless, efforts to develop better algorithms for the early detection of ovarian cancer currently represent an active and vital area of medical research, and CA125 determinations figure in most of the approaches proposed. For example, a large study involving 120,000 postmenopausal women is being coordinated at St. Bartholomew's Hospital, London. The study is focused on screening for ovarian cancer using CA125 determinations as the primary test, with sonography as a secondary test. Preliminary reports of this work in progress are encouraging.^6,7 Sophisticated algorithms involving longitudinal CA125 measurements also hold out considerable promise for detecting ovarian cancer at a relatively early stage of the disease process.^8,9

Assays for CA125
CA125 was originally identified by a monoclonal antibody that was raised against an ovarian cancer cell line and reactive with ovarian carcinoma. The marker is poorly characterized, and its biological function remains unknown. CA125 is a glycoprotein, aggregated to high-molecular-weight complexes (over 200 kDa), with a carbohydrate content of about 24 percent. Based on crossinhibition studies with 26 different monoclonal antibodies, the First Workshop on Tissue Differentiation of the International Society for Oncodevelopmental Biology and Medicine (ISOBM) concluded that CA125 comprises two major antigenic domains: one OC125-like, and the other M11-like.¹⁰

International standardization for CA125 assays has yet to materialize. Moreover, absolute values can vary markedly from one manufacturer to another, and across assay formats, even when the same antibodies are used.^11,12 Accordingly, both reference range studies of well-defined populations and studies of individual patient trajectories throughout the various phases of disease and treatment are extremely important in the evaluation of CA125 assays.

DPC's OM-MA assays
Assays for the quantitative measurement of CA125 in serum are available for both the IMMULITE® and the IMMULITE® 2000 automated chemiluminescent analyzers. (Outside the US, a manual immunoradiometric assay, IRMA-Count® OM-MA, is also available.)

IMMULITE OM-MA and IMMULITE 2000 OM-MA each received FDA clearance for use as an aid in monitoring the response to therapy for patients with epithelial ovarian cancer, and in detecting residual ovarian cancer in patients who have undergone first-line therapy and would be considered for diagnostic second-look procedures.

Both assays use a murine monoclonal antibody for capture and a rabbit polyclonal antiserum for detection. The monoclonal antibody, which accounts for the specificity of these assays, recognizes a repetitive protein determinant expressed in the protein core of the CA125 antigen. This antibody has specificity for an epitope overlapping with, or very near to, the M11 domain. The polyclonal antiserum, having been affinity-purified against CA125, is reactive with multiple epitopes on the antigen.

These automated, chemiluminescent assays have been subjected to extensive testing aimed at establishing expected values for normal reference groups and patterns for women with ovarian cancer followed longitudinally throughout the course of their disease.

Reference range studies
As part of a multicenter reference range study of DPC tumor marker assays, IMMULITE OM-MA values were obtained on 474 nonpregnant women, all in apparent good health based on a questionnaire. Figure 1 shows the results, plotted against age. Disregarding this parameter, analysis yielded 95th and 97.5th centiles of 18 kU/L and 24 kU/L, respectively.13 (These are indicated by horizontal dashed lines in both Figure 1 and Figure 2.)

For an enlarged view, click on the image.

In a separate study of CA125 during the menstrual cycle, 27 normal volunteers had blood sampled daily throughout one complete ovulatory cycle. The subjects were in apparent good health, with no evidence of endometriosis, and demonstrated normal levels for several reproductive hormones throughout the cycle under study. Results obtained by the IMMULITE OM-MA assay are plotted as a function of cycle position in Figure 2. (The horizontal axis ranges from the beginning of the follicular phase on the left, to the end of the luteal phase on the right, with cycle days for each woman normalized to a common phase length.)

Overall, the results from the ovulatory cycle study show a distribution comparable to the distribution in the multicenter study described above. (Here the 97.5th centile is 20 kU/L for the cycle as a whole, that is, when analyzed without regard to cycle position.) But it is evident from the graph that cycle position is relevant to the interpretation of CA125 results: elevations amounting to as much as twice the highest levels encountered in the luteal phase can occur during the early follicular phase.

For an enlarged view, click on the image.

The orange and lavender symbols represent the daily results for two individuals in this study with contrasting trajectories. In one case, the CA125 levels are relatively low, and show little variation throughout the entire cycle, though there appears to be a slight modulation towards higher values beginning at midcycle. In the other, CA125 levels are strikingly elevated in the early days of the cycle, but subside back into the reference range by late follicular phase. (Compared to the first subject, the midcycle fluctuation for the second subject is somewhat more pronounced.)

Although the association between menstruation and elevated CA125 levels has been noted in the literature,2,14 this study of complete, normal ovulatory cycles provides detailed insight into the phenomenon, though further questions remain to be answered. For example, only some of the subjects in this study showed menstruation-associated increases high enough to cause misinterpretation relative to the assay's upper reference limit; but since each subject was followed throughout just one ovulatory cycle, it cannot be determined from these results whether early follicular phase elevations are likely to be observed on a regular basis for the same subjects.

Longitudinal studies
In an extensive study of the IMMULITE OM-MA assay at two independent clinical sites, longitudinal determinations on 30 ovarian cancer patients showed a strong association between the clinical course of the disease and the levels of CA125 measured.¹⁵ Decreasing CA125 levels correlated closely with episodes of clinical regression, while rising CA125 levels correlated with periods of clinical progression.

For an enlarged view, click on the image.

Figure 3 displays one example from this series. Elevated CA125 levels were observed during surgical removal of the tumor. The levels decreased postoperatively and continued to decline throughout subsequent chemotherapy (CTX) cycles before there was clinical confirmation for no evidence of disease (NED). CA125 levels remained low for the next two and a half years, but then showed a pattern of increase before recurrence could be clinically confirmed. A different chemotherapy regime was started, which had the effect of lowering CA125 levels; and this was followed by clinical confirmation of NED. The chemotherapy cycles continued with no further recurrence being clinically detected during the course of this study. Other examples from this series showing the correlation of CA125 measurements with disease activity were presented at the 50th Annual Meeting of the American Association for Clinical Chemistry (AACC), and subsequently published. ¹⁶

Conclusion
Assays for CA125 (OM-MA) have an important and well-established role in the treatment of ovarian cancer. They also figure in the most promising algorithms which have been proposed for identifying ovarian cancer at an early, more effectively treatable stage. Studies of the IMMULITE OM-MA and IMMULITE 2000 OM-MA assays have demonstrated their suitability for use in contexts where CA125 measurements are appropriate, and have already helped to elucidate the relevance of cycle position to the clinical interpretation of CA125 levels in premenopausal women.

References

1. Oriel KA, Hartenbach EM, Remington PL. Trends in United States ovarian cancer mortality, 1979-1995. Obstet Gynecol 1999;93:30-3.

2. Masahashi T, Matsuzawa K, Ohsawa M, Narita O, Asai T, Ishihara M. Serum CA 125 levels in patients with endometriosis: changes in CA 125 levels during menstruation. Obstet Gynecol 1988;72:328-31.

3. Mol BWJ, Bayram N, Lijmer JG, Wiegerinck MAHM, Bongers MY, van der Veen F, et al. The performance of CA-125 measurement in the detection of endometriosis; a meta-analysis. Fert Steril 1998;70:1101-8.

4. de Bruijn HWA, ten Hoor KA, van der Zee AGJ. Serum and cystic fluid levels of soluble interleukin-2 receptor-alpha in patients with epithelial ovarian tumors are correlated. Tumour Biol 1998;19:160-6.

5. de Bruijn HWA, van der Zee AGJ, Aalders JG. The value of cancer antigen 125 (CA 125) during treatment and follow-up of patients with ovarian cancer. Curr Opin Obstet Gynecol 1997;9:8-13.

6. Jacobs I, Davies AP, Bridges J, Stabile I, Fay T, Lower A, et al. Prevalence screening for ovarian cancer in postmenopausal women by CA 125 measurement and ultrasonography. Brit Med J 1993;306:1030-4.

7. Jacobs I. Overview - progress in screening for ovarian cancer. In: Sharp F, Blackett T, Berek J, Bast R, editors. Ovarian cancer 5. Oxford: ISIS Medical Media, 1998: 173-85.

8. Skates SJ, Xu FJ, Yu YH, Sjövall K, Einhorn N, Chang Y, et al. Toward an optimal algorithm for ovarian cancer screening with longitudinal tumor markers. Cancer 1995;76:2004-10.

9. Skates S, Jacobs I, Knapp R. Quantifying risk of ovarian cancer using longitudinal CA125 levels. In: Sharp F, Blackett T, Berek J, Bast R, editors. Ovarian cancer 5. Oxford: ISIS Medical Media, 1998: 187-97.

10. Nustad K, Bast RC Jr, O'Brien TJ, Nilsson O, Seguin P, Suresh MR, et al. Specificity and affinity of 26 monoclonal antibodies against the CA 125 antigen: first report from the ISOBM TD-1 workshop. International Society for Oncodevelopmental Biology and Medicine. Tumour Biol 1996;17:196-219.

11. van Kamp GJ, Verstraeten RA, Kenemans P. Discordant serum CA 125 values in commercial immunoassays. Eur J Obstet Gynecol Reprod Biol 1993;47:245-51.

12. Davelaar EM, van Kamp GJ, Verstraeten RA, Kenemans P. Comparison of seven immunoassays for the quantification of CA 125 antigen in serum. Clin Chem 1998;44:1417-22.

13. Sibley PEC. IMMULITE tumor marker assays; multicenter reference range data for Diagnostic Products Corporation kits [technical report]. Los Angeles: DPC, 1999. Document No. ZB148-C.

14. Meden H, Fattahi-Meibodi A. CA 125 in benign gynecological conditions. Int J Biol Markers 1998;13:231-7.

15. Data on file.

16. Witherspoon LR, Lohmann TP, Lapeyrolerie T. IMMULITE OM-MA for the determination of CA125 [abstract 146]. Clin Chem 1998;44(S6):A35. Poster presentation at the American Association for Clinical Chemistry National Meeting. Reproduced in: 1998 Selected scientific posters and abstracts. Los Angeles: Diagnostic Products Corporation, 1999: 10-15. Document ZB183-A.